The reported upgrade paths are similar to those for going from an older OS to Win7, although the Win8 upgrades are a little more convenient.
XP, Vista, and Win7 were all stated as qualifying the use of a Win8 upgrade license. Only Win7 would allow an upgrade-in-place, and that only for specific versions. For XP and Vista owners, a clean (custom) install would preserve files and (in some cases) settings.
The Win7 upgrade required XP or Vista. Only Vista allowed an upgrade-in-place. A custom installation wiped everything (or moved it to windows.old). Windows 2000 users (not common) were not supported. 95, 98, and ME users, ditto.