Force exsisting mail

[admin at kclinux.net]

For some reason, I cant find an answer to this on the web.  Maybe it's not
possible?
 
I have a user that has a server with just over 1200 emails in root's email.
(About 4 are generated per day by the system.)
 
The user just created an alias for root so all the email will forward to his
email address.
 
How can I force all of those emails that already exsist in root's mailbox to
go to that alias address?