I'll need to know the errors you're getting when you try to DCPROMO down to a member server. Ordinarily, the process proceeds without problems. The most common problems involved DNS, so make sure the server can see the DNS server and resolve the name of another domain controller.
Is the other domain controller in the same site?
Another common reason for demotion problems is FSMO transfers. You may want to transfer the FSMOs from the server manually before trying to demote it.
Unfortunately, there is no way to force the machine to demote if it
doesn't want to. You'd have to reinstall the operating system.
This was first published in September 2001