How to Rename a source object during a project coexistance. (4254910)

To Rename a user’s Mailbox.

1. Rename source AD Attributes ie, Name, Displayname, etc.  for source users. These will be synchronized over to target by the Directory Synchronization Agent, or a migration session.

2. On the source copy the current/OLD Primary SMTP to Proxy addresses in Exchange to maintain mail routing to the old/original email address.

3. On the Target copy the current/OLD Primary SMTP to Proxy addresses in Exchange to maintain mail routing to the old/original email address.

4. On the Source, Set the new email address (primary SMTP for the new name) in Exchange

5. On the Target, Set the new email address (primary SMTP for the new name) in Exchange as Migration Manager (Per production design) will not synchronize over the SMTP address to the target "Out of the Box"

This KB explains the product functionality regarding primary SMTP, and a possible workaround, unfortunately, this impacts ALL Users.
https://support.quest.com/migration-manager-for-exchange/kb/39768