If you have a NFR copy of Windows NT 4.0 server, you can not upgrade it to a full retail version, even after performaing an update install from a full retail setup diskette set and CD-ROM.
When you use Control Panel / Licensing, you receive:
Not Available in NFR (Not for Resale) Edition of Windows NT Server.
When you use Control Panel / Licensing on an NFR copy of Windows 2000 Server, you do not receive this message.
If you purchase a full retail version of Windows 2000 Server, and perform an update install, you can remove the 10 client license limit using Control Panel / Licensing.
NOTE: If you increase the license limit in the NFR version of Windows 2000 Server, without first doing an update install with the full retail media, the change is accepted but the license limit is not changed.