New Post: Change Legacy DN address for email address

We have recently had to do 2 migration of exchange mail. Office 365 can only use one X500 address when users would reply to an old email.

We have 12 users with 1 months worth of mail where I would like to change the Legacy DN to our on premise DN. Thus all mail in the migration would be covered by the on X500.

I have just heard of MFCMAPI and belive that it could assist me. Would this be recommended?
Then what the best way to do this via a script or bulk change.

Any advice would be appreciated.

Regards