Migrate on-premise mailboxes to existant online users

cruizmct I'm happy to see somebody else is facing what I've been facing repeatedly with customers since COVID-19 (when everybody jumped on Teams quickly).

Here's an Exchange Team blog post about the topic of having a mailbox in both on-premises and in EXO at the same time:

https://techcommunity.microsoft.com/t5/exchange-team-blog/my-user-has-a-mailbox-both-on-premises-and-in-exchange-online/ba-p/846809

The recommended solution from Microsoft (or even better, the Exchange team) is to blow away the on-premises mailbox and then re-enable the user as a Remote Mailbox, then perform New-MailboxRestoreRequest from EXO PowerShell, restoring the soft-deleted on-premises mailbox directly into the live EXO mailbox.  The main caveat is users will need to create new Outlook profiles because they won't maintain their original on-premises mailbox's ExchangeGuid through this process.  But the big benefit is that they won't lose any of their Teams, SharePoint Online, OneDrive, etc. metadata.

If you go this route, you'll find that the New-MailboxRestoreRequest's end in "FailedOther".  Fortunately I've been through this many times now and have confirmed with the Exchange team and MS Support that the failure status is OK, as long as the Copy Progress shows the correct number of items having been copied.  I just (last Friday and over the weekend) had my Pull Request merged into the MS Docs page for this procedure, which explains this in more detail:

https://docs.microsoft.com/en-us/exchange/troubleshoot/move-mailboxes/mailbox-exists-exo-onpremises