change existing username style

I have accounts that I migrated from Ensim Pro. The usernames are in the form of usr@domain.tld. On at least some of the accounts I want to convert the username to the usr.domain style.

I realize I could do this by deleting and recreating the user, but I don’t want to loose the mail in their box.

Any thoughts on how to change an exiting username style or how to transfer the mail?