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#11
I have seen claims that you can clean install windows 10 and use an upgrade licence to activate it.
I guess this works fine because you can certainly upgrade from 8.1 previously upgraded from Windows 7 (done it myself).
I assume it works in a clean install, because MS only checks the key (as it would in an upgrade), but does not do any extra checks to see if you had an original full Windows 7 licence.