Switch to new server and forward existing mail from old server

[Clarence Brown]

Hi all:

I've tried googling, and found a ton of stuff, but nothing specific to 
my question. I've got an ancient postfix server with a couple virtual 
domains, been chugging along solid and stable for years.

Want to move to a new server at a different IP, Have the old server 
forward / transfer any mail collected in it's mail boxes to the new 
server, and not have the users have to make any changes to their email 
clients or loose any email. Is there any way to tell the OLD Postfix to 
grab a user's mail box and forward all the messages in it to that user 
at the NEW Postfix?

I'm concerned that if a user last checked email Friday afternoon at 
4:00, and I do the switch over say Saturday Night, any email collected 
in the interim will be stuck on the old server without manual intervention.

Thanks Cla.