Deleting the mailbox and creating a new one is not an option since I need to preserve the content.
From your comment, is the user storing a lot of mail on the server and using IMAP access?
A users mail is stored in various folders under /home/AccountName/mail/DomainName/User/ if using maildir, or in a single file if using mbox.
I would probably:
1) Create the new email address
2) Temporarily set a forward on the old mailbox to point to the new mailbox
3) Depending on the format, either copy the files over or cat the two files together.
4) Delete the old account
Its easy enough to create a test email account and go through the process before working with the live account.