this is nothing real special and there are a lot of Whitepapers on MS
is a good place to start.
Install new BDC for Domain with NT 4.
Apply Servicepack 6a.
Promote the server to PDC.
Unplug former PDC from network for fallback capabilities.
Upgrade the new PDC with Windows 2003.
Fix minor behaviurs you may have.
Best greetings from Germany