In an NT domain, if you promote the BDC to PDC, then the PDC demotes to a BDC. If you promte a 2K member server to DC status you in a sense create a new and seperate domain, other than the NT domain. That said, in your case I would do the following, replicate before doing this, 2K is used for to identify new computer):
1. Promote the BDC to PDC, thus demoting the current PDC to BDC status
2. Promote the new NT PDC to 2K
3. Promote the 2K member server to 2K DC status
4. Promote the NT BDC with Ex. to 2K (or leave it up to you, if you do promo it make sure you have Exchange with SP3 installed)
If your NT servers can't handle 2K then do the following:
1. Promote the 2K member server to BDC (basically install NT on it and make it a BDC, (2K is used for identity))
2. Take the NT BDC offline for backup purposes
3. Promote the (2K) BDC to PDC, thus demoting the current NT PDC to NT BDC status
4. Promote the new (2K) NT PDC to 2K
5. Leave your NT BDC's as is....
I hope that answers your question.
Hewissa
MCSE, CCNA, CIW