Not surprisingly, users may not downgrade to the still-used-but-slated-for-retirement Windows XP.

Downgrade rights -- which let customers replace a newer version of Windows with an older edition without paying for two copies -- are available only in Windows 8 Pro. That fits with previous practice: Only Windows 7 Professional, for instance, was allocated downgrade rights.

"Instead of using the Windows 8 Pro software, you may use one of the following earlier versions: Windows 7 Professional or Windows Vista Business," states the software license agreement for the version of Windows 8 Pro that will be installed by computer makers (OEMs) on new PCs.

Windows XP Professional, which was one of the allowed downgrades for Windows 7 Professional, was not named. Windows XP will fall off Microsoft's support list in April 2014.

One licensing expert noticed XP was AWOL.