decommision resource forest and install Exchange 2016 in account domain with existing hybrid

Hi Olaf,

Based on my expierience doing a lot of account/resource forest migrations when hybrid is already in place: Do first the consolidation within your forests and then go hybrid to your tenant before moving any users to Exchange Online.

Another possible solution is to migrate all users to Exchange Online, decomissioning Exchange 2010 and configure Exchange 2016 for hybrid and offboard users if necessary.

The problem with already migrated users from Exchange 2010 to Office 365: you "must" normallly use Prepare-MoveRequest prior migration users to Exchange Online to create a MEU object in the new Exchange 2016 forest. It is not supported to write Exchange attributes with ADSIEDIT or other tools without using Exchange.

If you setup the Exchange 2016 hybrid environment, your already migrated users will be a "RemoteMailbox, Privisioned". Your currently migrated mailboxes from Exchange 2010 are "RemoteMailbox, Migrated". And, if you want to offboard users during "reversy hybrid" you have to export the MailboxGUID from the Exchange Online mailboxes to your on-premise MEU object.

Long speak short: firest consolidate your on-premise Forests with a cross-forest migration and then go hybrid to your tenant. This is the "supported" and easiest kind of migration. Of course, you can copy all Exchange attributes with PowerShell or FIM (except some attributes like HomeMDB, ExchangeVersion, etc.) after your hybrid switch to the Exchange 2016 forest, but this requires a lot of planning and some other tools for the attribute flow between the forests.

Cheers,

Dominik