Decommision a 2000 server running AD

I need to decommission a 2000 server that is acting as a domain controller and is running Active Directory. I have put into place a new 2003 server (also running AD) and created a new domain. The old server is also running Exchange 2000, which I would like to keep in place. My idea is to decommission the 2000 server(as a domain controller) and keep it as an Exchange server and join it to the new domain. Any help would be greatly appreciated. Thanks.