Leave the first server's Exchange services up for a few days until all Outlook clients have connected once (their MAPI profiles will be re-written to point to the new Exchange Server). Finally, shut down the Exchange services on the first machine for a few days once you think everything is migrated over. Test free/busy, offline address book (OAB) generation and organizational forms (if you have them). Then, once you're sure everything is working, uninstall Exchange from the first server.
As always, backup, backup, backup ... and test in a lab first.
Hope that helps!
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