On completion day, nobody expects the IT integration to be finished. They do expect people from both businesses to be able to meet, email and work together, and they expect nothing to get less secure. That is what Day 1 is for.
Before completion
Confirm what can be shared before the deal closes, and what has to wait. Get the acquired business's tenant, domain and admin details as early as the deal allows. Agree who approves IT changes in the acquired business on Day 1, and who pays for them.
Security before trust
Check the acquired tenant before connecting anything to it. Look at admin accounts, MFA coverage, Conditional Access, guest accounts, forwarding rules and signs of compromise. Connecting two tenants extends trust in both directions, so fix the worst gaps first.
Working together on Day 1
Free/busy sharing so meetings can be booked across both businesses. A shared address list so people can find each other. Teams federation or cross-tenant access so chat and meetings work. Access for the acquired business to the handful of shared sites or systems they need immediately.
People and communication
Tell staff what changes on Day 1 and what doesn't. For most people in the acquired business, the answer is very little: same laptop, same email address, same tools for now. Give them one place to ask questions and one support contact.
Support
Decide who supports the acquired business's users from Day 1: their existing IT team or provider, yours, or both. Write down the escalation path between them, and make sure the service desk knows about the integration plan.
Decisions that can wait
Email address changes, tenant migration, device re-enrolment, application consolidation and licence changes can all follow in the Day 100 plan. Rushing them onto Day 1 is how integrations create outages.
After Day 1
Within the first weeks, complete the inventory, agree the migration plan and dates, and confirm any transitional arrangements with the seller. The Day 100 plan turns that into waves.
Working to deal dates? Tenant migration readiness
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