Retire domain controller

This should cause no problems, as long as you have a functioning PDC remaining. Before you can demote the PDC, you must have an available BDC that you can promote.

Unlike Win2K, where all Domain Controllers are essentially co-equals (all have writable copies of Active Directory), NT4 BDCs have read-only copies of the SAM database. Therefore, assuming that no other services are being run on that server, you need only remove the BDC from the domain (using Server Manager on the PDC), then do what you need/want to do with that box.

be sure to check your WINS configs. removing a server with WINS and not re-assigning it to a new server can result in tons of support calls. if you are using DHCP make sure the DHCP server has the re-assigned IP address for services that may be moved once the PDC roles have changed.

You should also make sure that your older PDC is not the Export server for LAN Manager Replication

And since you planing a migration to windows 2000 please read the following Microsoft kb article:
http://support.microsoft.com/?kbid=248358

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