For more in-depth information and guidance, head to our Help Center and check out Collaboration Migration Best Practices. As with any migration project, pre-planning is pivotal.
Prep the destination ahead of time.
Ensure the source and destination have the same configuration. All users should be set up and licensed on the destination prior to the migration. User setup may take up to 24 hours in the tenant.
Create administrator accounts dedicated to the migration.
Create these new accounts on both the source and the destination. Turn off Active Directory Federation Services and Multi-Factor Authentication on both accounts; if enabled, they prevent the migration from proceeding. Create these accounts at least 24 hours prior to migration testing, as permissions take time to sync.
Allow ample time for the migration.
Large Teams migrations are tricky, and user collaboration can greatly impact the amount of data that needs to be migrated. Using Teams primarily as their document repository can introduce serious delays. Users cannot collaborate during a migration; bigger migrations are best done on a weekend to allow uninterrupted data transfer.
Do a test migration first.
To gauge the amount of time the full migration could take, select a team that is representative of your average channel size. This serves as a proof-of-concept.