Read EPC Group's 2026 enterprise guide to M365 tenant-to-tenant migrations this morning....

Read EPC Group's 2026 enterprise guide to M365 tenant-to-tenant migrations this morning. Solid reference, but it reminded me how much of a migration's success lives in sequencing decisions that never make it into a guide. Entra ID first, then Exchange, then SharePoint and OneDrive - fine on paper. What breaks in practice is everything downstream of identity: conditional access policies that don't survive the cutover, guest accounts pointing at the old tenant, service principals no one documented, MFA registrations that have to be re-enrolled at exactly the wrong moment for the business. On the M&A side, the harder question is timing. Legal close rarely lines up with an IT-clean cutover window. You run coexistence longer than anyone wanted - shared calendars, cross-tenant mail flow, GAL sync - and each extra week is another week of tickets. Guides tend to treat tenant-to-tenant as a technical exercise. After running a couple hundred of these, my honest take is that it's a change management exercise with a technical component. If the business side hasn't agreed on what 'done' looks like - which users, which data, which apps, on what date - the tooling won't save you. What's the longest coexistence window you've had to operate through?