The command that should normally allow you to change the serial number of Windows is:

     sysprep -nosidgen

The sysprep utility is part of the Resource Kit.

It is not certain whether there are some versions (OEM or other) where this would fail and even going back to the old number might fail, so do have a complete backup before you try.

Another way to change the serial number is to activate by telephone, rather than through the Internet. There you get a choice of entering a new number. After this one-time action you can activate through the Internet again, using the new number.