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I have been preparing a site that is running Exchange 2013 in a domain(e.g. @domain.local/@domain.org), to have the mailboxes migrated to an Exchange Online setup. The problem I am now realizing is that the O365 setup already has active users that are setup (e.g. user@ab90.onmicrosoft.com) where those accounts have been used to install O365 applications on the site's workstations. I am coming in on this after a site "power-user" has setup these accounts and installed O365 on 20+ workstations.

My concern is that the cutover migration process I am planning for (https://docs.microsoft.com/en-us/Exchange/mailbox-migration/cutover-migration-to-office-365?redirectSourcePath=%252farticle%252f365-9496e93c-1e59-41a8-9bb3-6e8df0cd81b4) would create new mailboxes/users on the O365 side and blow past their license allowance, which is already used up by the manually created O365 users.

I not sure if I should proceed: 1. delete the O365 users, thus freeing up licenses, and migrate the mailboxes/users from the on-premise AD/Exchange. This would I assume require re-activation of the existing O365 installations at the site. I would then setup the ADConnect sync tool to get local AD and Exchange online/Azure synced. (this seems like what I should do) OR 2. Rename the O365 users from e.g. user@ab90.onmicrosoft.com to user@domain.org, get their on-site O365 installations using that account name, then import the data from Exchange 2013 to the existing mailboxes on O365 (this part I'm not sure on and MS has not been helpful in answering this question) OR 3. Some combination of the above or other suggestions.

Thank you!

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I would recommend moving the mailboxes to the cloud first, then change the license and inform the users that they need to log off / on again in the Office suite.

When migrating to Office 365 you actually have about 30 days for migrated mailboxes before you need a license on them. So you could just migrate them over, when you have done a cutover, move the licenses afterwards.

Do remember that any data users have in places like OneNote and OneDrive will not be moved over with this kind of change.

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