The "official" way to do this is to insert the original product CD and perform an in-place upgrade, i.e., reinstall Windows on top of itself. You'll need to re-apply all service packs and hotfixes after you do this. You can also call Microsoft directly and ask for help in this regard. They are usually very helpful.
There is another method which I have not tested, which involves hacking the Registry, and which does not seem to be well-supported in Windows 2003 Server:
- In the Registry, go to the key HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\WINDOWS NT\CURRENT VERSION\WPAEVENTS.
- Edit the key OOBETIMER. Change at least three of the binary values therein.
- Close REGEDIT.
- Go to Start | Run, and type c \windows\systems32\oobe\msoobe.exe /a
- This will launch the Product Activation screen. You should be able to specify a new product key on the second screen by selecting "Change Product Key."
This was first published in January 2006