A. You cannot change the role of a NT server, you will need to reinstall NT.
As an alternative, you can use a 3rd party utility called U-Promote, http://u-tools.com/UTools/UPromote.asp however this works by making registry changes that would render your system unsupportable by Microsoft and may lead to problems later in the servers life time, for example moving to Windows 2000.In Windows 2000 you can promote a server to a domain controller by using DCPROMO.EXE.