As I see from here: http://windows.microsoft.com/en-US/windows-8/terms-conditions
There is no requirement for buying a previous version of windows, it only says that you should have running one of those. I mean, you can use evaluation version or unactivated version which are legally accessible. And do this windows 8 pro upgrade. Also here: http://windows.microsoft.com/en-US/windows/buy it says: "To install Windows 8 Pro, customers must be running Windows XP SP3, Windows Vista, Windows 7, Windows 8 Consumer Preview, or Windows 8 Release Preview." If I'm not mistaking Windows 8 Consumer Preview and Windows 8 Release Preview are freely available.
So what I'm asking is, is it said anywhere that I must own previous version of windows to windows 8 pro upgrade?
Please don't confuse this upgrade with another available only for a PC bought with preinstalled windows 7.