Downtime comes from skipped discovery.
List every mailbox, shared mailbox, archive, domain and application that sends mail through the current system before anything moves. Surprises at cutover are almost always items discovery missed.
Pilot before you commit the company.
Move a small group first and run them in production: mail flow, mobile devices, delegates, calendars. The pilot either validates the plan or shows exactly what to fix while the rest of the company is unaffected.
Cutover is a scheduled moment, not a weekend hope.
The DNS cutover is planned for a defined window outside business hours, with mail flow verified on both sides and a documented rollback. Done this way, users simply open their mail in the new tenant on the next working day.
Plan the first week after.
Keep a support window for stragglers, verify SPF, DKIM and DMARC on the new platform, and retire the old system only when nothing depends on it. The handover document matters as much as the move.
General purchasing guidance. Verify the current product terms and the conditions of your specific offer before purchasing.