Are both servers in the same org and admin group ?
Are all the users off the old server ?
Have you moved your connectors off the old server ?
Do a look on the MS knowledgebase for “how to remove the first server”. You will probably have to move your free busy and offline address book folders to the new E2K3 server and then uninstall the old server.
Also if you rename or rebuild the old box, your outlook clients might hang trying to contact it. Solution is to recreate all your mapi profiles.
MS have an arcticle in their knowledgebase on how to remove the first server in the site ( for 5.5 ) I presume theres a similar one for SBS. You can also lookup “going to native mode Exchange 2003″.