You can run any mix of 2000 and 2003 servers without issue. You need to run adprep /forestprep and adprep /domainprep from the 2003
CD first to upgrade the active directory schema to accomodate 2003 Server. (If you are running Exchange Server you might do a little
reading on that first.)
These schema changes need to replicate fully, and from experience I can tell you that you need to make sure (via netdiag and dcdiag)
that your domain is correctly configured and running without any errors. And of course be sure to have a full AD & system state
backup before you do anything like this.
You will have to run the domain in 2000 mixed or native mode, and so many of the feature enhancements of 2003 Server will not be
available.
This will give you some more details:
http://support.microsoft.com/?id=325379 This article is for upgrading 2000 Server to 2003
Server, but the domain issues are the same either way.
Steve Duff, MCSE, MVP
Ergodic Systems, Inc.