Exchange migration - User existed in the target with mailbox, overwritten by source with PrepareMoverRequest.ps1 and merged with ADMT

Hi,


Exchange 2010 Cross-Forest migration to Exchange 2013, user with mailbox already existed in the target domain unfortunately the source AD object was edited with prepare-MoveRequest.ps1 and merged using ADMT. 

Now the user cannot access his email account, keeps getting username password prompt, Iv'e change his password a few times to see if this was an AD specific issues before learning he had a previous account in the target domain.

How would I go about re-associating the users AD account with the correct Exchange Mailbox?

Thanks in advance!