Exchange Hybrid - Migrate large archive mailbox

Copper Contributor

We have a hybrid Exchange environment with Exchange Server 2016. We assigned our users Microsoft 365 Business Premium licenses that include Exchange Online (Plan 1) and Exchange Online Archiving. We have migrated all the users but one. This guy has primary mailbox of 40GB and a 110GB archive. The archive quota in Exchange Online is 100GB initially but since we have Exchange Online Archiving license, we can use the 1.5TB quota for the archive mailbox. The problem is that in order to use this, auto-expanding archiving must be enabled on the mailbox. And we can't enable it before migrating the mailbox because the mailbox is still on-premise. We haven't tried migrating the mailbox in it's current state but I guess it will fail. So basically I can think of three hypothetical solutions but I don't know if any of them will work.

The first one is to export approximately 20GB from the archive mailbox to a .pst file. Migrate both the primary mailbox and the archive, enable auto-expanding archiving and then import the .pst file. This will probably work but it's a lot of manual labor on the user's computer which we want to avoid.

The others are to migrate separately the primary mailbox and the archive. What happens if we migrate the primary mailbox and the archive stays on-premise? Will we be able to enable auto-expanding archiving then? Or if we move 20GB from the archive to the primary mailbox and then migrate only the archive to Exchange Online, will we be able to enable auto-expanding archiving then?

Is there some general procedure that addresses such cases? I really doubt we are the only ones that hit this problem.

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