We’ve just installed a new Exchange 2003 server at work, and we’ve been busy migrating mailboxes and public folders from the old server to the new one. Something to note is MS’s article on how to do this if you plan to remove the old server when you are finished.
One thing to note that we missed, make sure that the SMTP connector on both server remains enabled (we turned it off on the old server to start shutting down services). It you disable it then ALL sync’ing between the servers stops working. If you want to stop outgoing email, then remove the old server from the bridgehead list.