Modify ProxyAddresses of Office 365 users without Exchange Online
As part of a Office 365 tenant rebuild, I had to move a custom domain to the new Office 365 tenant. The old tenant was not needed anymore, and the customer had to move to a Non-Profit tenant for compliance reasons. So the migration itself was no big deal:
disable AzureAD sync change UPN of all users remove the domain connect the domain to the new tenant setup a new AzureAD sync assign licenses time for a beer That was my, honestly, naive plan for this migration.