The domain and all data is already in our tenant. the back story is we bought oldcompany and brought them into our tenant. Now that oldcompany is merging with newcompany and thus will be getting newcompany email address. Further as an example: firstname.lastname@example.org has 10+ years of data in OneDrive, emails, etc. However when newcompany first took over it gave Me a email@example.com email and has also been using that. Now that oldcompany is folded into newcompany, we no longer needs firstname.lastname@example.org and wants all data to be moved to email@example.com
Gosh, I hope all that made sense!
Thank you in advance for any insights or thoughts!
from an Exchange point of view i'd look at a 3 party migration tool. I have used Bit Titan migrationwiz multiple times for this exact task - you can create a mapping that has source and destination email addresses and import that into the project specifying the source and destination endpoints and run the migration.
For the migration of OneDrive data there are some migration tools could be used. but a really simple method if permitted by policies would be to sign into OneDrive with both accounts and copy from the old OneDrive account to the new OneDrive