The First 100 Days After an Acquisition: A Phase-by-Phase Timeline

You've done your discovery. You have a roadmap. Now it's time to execute.

The first 100 days of integration are when most of the real work happens. It's also when things are most likely to go wrong.

Research from Global PMI Partners shows that 92% of integrations succeed when synergies are explicitly validated and tracked from the start. The first 100 days is your window to establish that discipline and to avoid the drift that derails so many deals.

According to PwC, the "golden period" for achieving higher ROI is within the first year. What you do in the first 100 days sets the trajectory for everything that follows.

This article breaks down the timeline into phases with clear priorities for each, so you know what to focus on and when.

This article is adapted from Chapter 5 of The Roll-Up Integration Playbook, our free guide to post-merger technical integration.

A Note for European Operations

If your acquisition is in Germany, France, or other countries with works council requirements, your timeline needs adjustment.

In Germany, companies with more than five employees can form a Betriebsrat (works council) that must be consulted on restructuring, significant IT changes, and headcount decisions. In France, the Comité Social et Économique has similar powers.

These consultations can add weeks or months to your timeline. A restructuring that takes two weeks in the US might take two months in Germany. Factor this into your planning from the start.

Day 1: Setting the Tone

Day 1 matters more than you might think. It's when the acquired team forms their first impressions of what life will be like under new ownership.

Get it right and you build trust that makes everything else easier. Get it wrong and you're fighting an uphill battle for months.

Communicate Early and Clearly

The acquired team is anxious. They've heard rumours, worried about their jobs, and wondered what "integration" means for them personally. Silence from leadership makes this worse.

On Day 1, communicate:

• Who you are and how to reach the integration team
• What's happening in the first few weeks (and what's not)
• What's staying the same (at least for now)
• How decisions will be made and communicated going forward
• Who to ask if they have questions or concerns

You don't need to have all the answers. But you do need to establish that you're paying attention, that there's a plan, and that people won't be blindsided by changes.

Remember: 47% of acquired employees leave within the first year. How you communicate from Day 1 directly affects whether you keep the people you need.

Secure the Basics

Day 1 is when you lock down security fundamentals:

• Privileged access audit
• MFA on email
• Endpoint protection verified
• Backups confirmed

Don't skip this because you're busy with other things. A ransomware attack in month two will cost far more than the time spent on basic hygiene.

For the detailed security checklist, see The Post-Acquisition Integration Audit.

The Day 1 Email Question

Some operators migrate email and identity on Day 1 itself. The logic is sound: email is a security perimeter, and you want to control who has access from the moment you own the company.

Others prefer to wait, doing email migration in the first few weeks after discovery is complete. The logic here: email migration affects everyone, and doing it before you've built any trust can feel heavy-handed.

Both approaches are valid. If security is your primary concern, Day 1 email migration makes sense. Just be prepared for the disruption it causes and communicate clearly about what's happening and why.

What NOT to Do on Day 1

Don'tWhyAnnounce major business system changesEven if you know the CRM is getting replaced, Day 1 isn't the time. Let people settle first.Rush migrations you're not ready forEmail/identity can be a Day 1 move if prepared. CRM or ERP before discovery is complete is how things break.Make promises you can't keep"Nothing will change" is almost never true. Better to say "here's what we know so far."Ignore the acquired leadership teamThey're watching closely to see if they still have a role. Involve them early.Disappear after the announcementBe present, be available, answer questions even if the answer is "we're still figuring that out."

Days 1-30: Stabilisation Phase

The first month is about completing your discovery, building relationships, and setting up the foundations for migration. You're not trying to transform the business yet. You're trying to understand it deeply and earn enough trust to make changes later.

Complete Your Discovery

Use the first few weeks to fill in the gaps now that you have full access:

• Finish the system inventory
• Complete department interviews
• Assess data quality in detail
• Map stakeholders and identify your champions and skeptics

Don't rush this. The quality of your discovery work directly affects how smoothly the rest of the integration goes. For the full discovery process, see The Post-Acquisition Integration Audit.

Deliver Quick Wins

Quick wins serve two purposes: they show progress to leadership, and they build credibility with the acquired team.

Examples of good quick wins:

• Unified email signatures and branding (if doing High-Touch)
• Access to platform collaboration tools (Slack, Teams, shared drives)
• Fixing obvious pain points that came up in interviews
• Clearing the backlog of IT tickets the previous owner ignored
• Setting up proper backup and monitoring

What makes a good quick win:

• Visible to the people affected
• Low risk of causing problems
• Can be completed in days, not weeks
• Demonstrates that you're listening and delivering

What's NOT a quick win:

• Major system migrations
• Process changes that require training
• Anything that could disrupt operations if it goes wrong

Establish the Financial Bridge

Your board and investors want consolidated reporting. They don't want to wait 90 days for it.

Even before you integrate finance systems, you need a way to get numbers into your reporting. The "financial bridge" is typically a structured export from the acquired company's accounting system, mapped to your chart of accounts, delivered on a defined schedule.

What you need:

• Agreement on reporting timeline (when do they submit, when do you consolidate)
• Mapping between their chart of accounts and yours
• A clear process for who does what
• A way to handle questions and discrepancies

This is temporary, but "temporary" might be 3-6 months while you plan the full ERP integration. Make it robust enough to rely on.

Build Relationships

Integration is a people exercise as much as a technical one.

• Spend time on-site if possible (or video calls if not)
• Have informal conversations, not just formal meetings
• Listen more than you talk
• Follow through on anything you commit to

Bring the teams together. Introduce counterparts across organisations: your finance lead meets their finance lead, your ops manager meets theirs. These relationships matter when you're aligning processes later.

Early introductions prevent an "us vs them" dynamic. When people know each other personally, they're more likely to collaborate on process changes rather than defending their turf.

Plan the Core Migrations

By the end of the first month, you should have detailed plans for your major migrations. Not "we'll migrate the CRM" but:

• Exactly what data is migrating and what's being archived
• Field mapping between source and destination
• Data cleanup required and who's doing it
• Migration approach (big bang vs. phased)
• Testing plan
• Training plan
• Rollback plan if things go wrong
• Timeline with key milestones

These plans get reviewed and approved before execution starts.

Days 31-90: Execution Phase

With discovery complete, relationships established, and plans approved, you're ready to execute the core migrations. This is the intense phase where most of the technical work happens.

Migration Sequencing

The order you migrate systems matters. Some migrations create dependencies for others. Some are lower risk and good for building confidence before tackling harder ones.

Typical sequence:

OrderSystemRationale1Email/IdentitySecurity perimeter, affects everyone but technically straightforward2CRMCustomer data unified, enables cross-sell and consolidated reporting3Finance/ERPConsolidated financial reporting, chart of accounts alignment4HR/PayrollEmployee data unified, benefits alignment5Operational SystemsIndustry-specific tools, often most complex

This isn't rigid. Your business priorities should drive the sequence. If finance reporting is your biggest pain point, ERP might come earlier. If the acquired company's CRM is actively losing deals, that becomes urgent.

For detailed guidance on each system type, see System Migration After Acquisition: CRM, ERP, Email, HR, and Operational Tools.

Parallel Running

For critical systems, don't flip the switch and hope for the best. Run the old and new systems in parallel until you're confident the new system is working.

How parallel running works:

• Both systems stay active
• Data is entered in one system and mirrored to the other (or entered in both)
• Reports are generated from both and compared
• When outputs match consistently, you cut over to the new system

When to use parallel running:

• Finance systems (you cannot afford errors in financial reporting)
• Any system where data integrity is critical
• Systems where users are skeptical and need proof the new system works

The downside: Parallel running takes more effort and extends timelines. Use it where the risk justifies the cost.

Tracking Progress

During execution, you need clear visibility into what's on track and what's not.

What to track:

• Milestone completion against plan
• Issues identified and their status
• Data quality metrics (records migrated, exceptions, errors)
• User adoption (who's using the new system, who's not)
• Risks that have emerged

How to track:

• Weekly status meetings with the integration team
• A simple tracker (spreadsheet is fine) that shows red/amber/green status
• Escalation criteria so problems get raised before they become crises

Don't let tracking become bureaucracy. The goal is visibility, not paperwork.

Escalating Issues

Things will go wrong. The question is how quickly you identify problems and how effectively you resolve them.

Signs you need to escalate:

• Timeline slipping with no clear path to recovery
• Data quality issues worse than expected
• User resistance that's blocking adoption
• Technical problems you can't resolve
• Scope changes being requested

Escalate early. The worst outcome is a problem that festers for weeks because nobody wanted to deliver bad news.

Communication Throughout

Keep stakeholders informed throughout execution. Don't go quiet and hope people assume everything is fine.

Weekly updates should cover:

• What was accomplished this week
• What's planned for next week
• Any issues or risks
• Decisions needed

Over-communicate during integration. People are anxious. Silence breeds rumours. Regular updates, even when the news is "on track, nothing major to report," build confidence.

Days 90+: Stabilisation and Handoff

The formal integration may be "complete" at Day 90, but the work isn't over.

Hypercare Period

Plan for a 2-4 week hypercare period after each major migration:

• Integration team remains fully engaged
• Issues get priority response
• Extra support available for users struggling with new systems
• Daily check-ins to catch problems early

Don't declare victory and move on the day after migration. The real test is whether the system works under normal business conditions.

Training and Adoption

Technical migration is only half the job. If people don't know how to use the new systems, or don't want to, the migration has failed regardless of how cleanly the data moved.

During the hypercare period:

• Identify users who are struggling and provide additional support
• Watch for people reverting to old tools or workarounds
• Gather feedback on what's working and what isn't
• Reinforce training on areas where adoption is low

For the full adoption playbook, see Why Systems Don't Fail — Adoption Fails.

Knowledge Transfer

Before the integration team moves on, ensure knowledge is transferred to the people who will run the business day-to-day:

• Documentation of what was done and why
• Training materials and recorded sessions
• Known issues and workarounds
• Contacts for ongoing support
• What to watch for in the coming months

The people who ran the business before should be able to run it after, without depending on the integration team.

Lessons Learned

After each integration, capture what worked and what didn't. This is how you build a repeatable capability.

• What took longer than expected?
• What surprises did we encounter?
• What would we do differently next time?
• What tools or templates should we create?

If you're doing multiple acquisitions, this learning compounds. Integration five should be significantly smoother than integration one.

Case in Point: 6 Months to 45 Days

Core BTS, a PE-backed IT services roll-up, found their first acquisitions taking up to six months to integrate. Each one felt like starting from scratch. After investing in a documented playbook with standardised processes and templates, they reduced integration time to 45 days, a 73% improvement.

Research backs this up: programmatic acquirers achieve 8.5% TSR growth compared to 3.7% for ad-hoc acquirers. The discipline of capturing lessons learned and building repeatable processes pays compound dividends.

The 100-Day Summary

PhaseDaysFocusDay 11Communication, security basics, tone-settingStabilisation1-30Discovery, quick wins, financial bridge, relationship building, migration planningExecution31-90Core migrations, parallel running, progress tracking, issue resolutionHandoff90+Hypercare, training reinforcement, knowledge transfer, lessons learned

Key Takeaways

• Day 1 sets the tone. Communicate clearly, secure the basics, and don't make changes you're not ready to support.
• The first month is about stabilisation, not transformation. Complete discovery, build relationships, deliver quick wins, and plan carefully.
• Sequence migrations based on dependencies and business priority. Email/identity first, then CRM, then finance, then HR, then operational systems. Adjust based on your specific situation.
• Use parallel running for high-risk systems. Don't flip the switch and hope. Run both systems until you're confident.
• Track progress and escalate early. Problems caught in week two are easier to solve than problems discovered in week eight.
• Over-communicate throughout. Regular updates build confidence and prevent surprises.
• Plan for hypercare and knowledge transfer. The integration isn't done until the business can run without the integration team.

Get the Full Playbook

This article covered the first 100 days execution framework. For the complete guide, including the integration level decision framework, system migration guidance, and ready-to-use checklists, download The Roll-Up Integration Playbook.

Download the Playbook

About PMI Stack

PMI Stack helps small-to-mid cap roll-ups unify systems, data, and workflows across their acquired companies. We specialise in the technical side of post-merger integration: data migration, system consolidation, and the change management that makes new tools stick.

If you're planning an integration and want to talk through your specific situation, book a free discovery call.

Statistics cited from Global PMI Partners, PwC, Bain & Company, and EY. For the full research compilation, see 50+ Post-Merger Integration Statistics (2026).