Like we saw last time around, some of the 7-to-8 upgrade rules are a bit Byzantine -- 7 Professional users can't step down to the non-Pro Windows 8 release, for example, and it's equally verboten to change language editions or move from 32- to 64-bit code without a full-scale reinstallation.
ENGADGET: Microsoft reportedly plans Windows 8 upgrades all the way from XP, wants us to just let go of the past