Hi Kelly,
It doesn't matter if you are running the Windows 2000 DC in the mode of
"Windows 2000 mixed", "Windows 2000 mixed" mode can support Windows Server
2003 based domain controller. Since all of the Windows NT 4.0 servers are
no longer a part of the domain, you may demote them and disjoint them from
the domain. After you finish the in-place upgrade the Windows 2000 DC to
Windows Server 2003 DC, you may consider raise domain and forest functional
levels in Windows Server 2003 to improve the domain functionality.
For more information, please refer to:
How to raise domain and forest functional levels in Windows Server 2003
http://support.microsoft.com/kb/322692
Hope this can be helpful.
David Shen
Microsoft Online Partner Support