Merging identical mailboxes from exchange 2003 to 2010

Question

I'm doing a test run of a server 2003 migration to exchange 2010. So far
everything is going ok and following the most recommended guides. One
question I have is doing this test run I'm moving the 2003 mailboxes to
exchange 2010. This is a very long process as some of these mailboxes
are over 4GB, in one case 20GB. My question is, if I take this new
server that is joined to the same domain as the cloned 2003 server and
plug it in to our production network with the main server, could I move
the mailboxes again to have all the updated mail? The database I'm
moving now is a snapshot from over a month ago so I'm wondering if
exchange can merge mailboxes that are actually identical but reside on
different server. I essentially have a copy User1's account from months
ago on 2010 now and User1's currently in use account on 2003. Will this
work to move again, and will it just overwrite the whole box or merge
the content and save time? I'm trying to minimize downtime so any advice
on how to do this most efficiently would be appreciated.