A Win 2k server can only be a PDC emulator. The PDC emulator role can be
transferred to another DC without re installing.
See:
HOW TO: View and Transfer FSMO Roles in the Graphical User Interface
If you are talking about removing the domain controller status from the Win
2k DC run dcpromo. This will remove AD and make it a member/standalone
server. It will also remove all users set up in the domain.