well, I am a bit late
but I have info about this. This is what we did at my work (about 100 users, plus 2 doamins)
Installl NT server in 2 computers. Set them up as Backup Domain controllers. Make sure the BDC replicated. Take one offline for rollback pourpouse (if needed). The other you will do an upgrade from NT to 2003. So, now you have one computer running as 2003 BDC, raise the level to primary.
Install your new box with 2003. Sinchronize it. Take offline the other NT DC (The original). Now, there should be no BT4 server Domain controllers (if there is more than one put them offline) Check everything is working. From now on, there is no tunring back...delete the entries for the NT boxes in the domain controller list. Before doing this is recomendable to have a 2003 backup Domanin Controller. The box you upgraded is fine, but I feel more confortable with a clean copy. If needed format one of the old NT DC and install 2003 into it, thought, for the money of the licence, you probably get a more powerfull box by buying a cheap server (with SATA RAID 0) and 2003 already installed into it.
Once all NT list have been erased, format the old NT DC (You don't want to turn them on by accident)
Done.
You need 2 2003 servers runnign as AD for run in native mode