To install Windows 8 Pro upgrade, customers must be running Windows XP SP3, Windows Vista, or Windows 7
Considering the quote, it would appear, that you need to be running one of the qualifying OSes. But for the Windows 7 release, it stated you had to have a previous OS installed and activated...
I have not yet purchased any Windows 8 product since I was looking for, but have not found a full retail version, except for the OEM.
Considering some of the changes that occurred when Windows 7 came out, if you have not forgotten, Microsoft can change what they will allow at any time. It appears, because of the price, they are trying to get Windows 8 on as many systems as possible, so they may be forgiving certain things, like a prior OS, to get folks to install this one.
If they are forgiving certain requirements, that might continue for a while, possibly the end of January, but no way to know. The fact they are not selling a full retail version means something. I suppose there will not be any version of Windows 8 beside the Pro version..so no bitlocker or language packs -- ever?
Edit: Looks like Bitlocker may be in the pro versions, and I suppose language packs would be an additional item as the Media Center is. I also found this statement. So if you have a prior OS activated, maybe they are just reading your computer to see if it has a prior license. Maybe we need to hear from someone with a new system and whether the upgrade will install on that system.
you can install the upgrade only on PCs that have a valid base license