This is how I moved all accounts off of an WinNTSBS 4.0 to Win2k DC. It required a rebuild on my domain controller, but was relatively fast, painless, and all data was moved and I didn't have to change the domain name. We will call the WinNT SBS 4.0 OLDDC and the Win2KDC NEWDC
Put WintNT on NEWDC, make it a PDC, and let it synch with the OLDDC. Disconnct NEWDC from the main network, and load WIN2K on it, Upon finishing, run DCPROMO to promote it to a domain controller. Demote the OLDDC to PDC, and bring the new Win2K DC up on the network. All your user, computers, and data should be intact.