I've had experience with an upgrade version of Windows 7 and the requirement for a clean install - upgrade from 32-bit Vista to 64-bit Windows 7. I have not tested this with Windows 8 and hoped Greg would do that for us. Instead he doesn't even show the screen on which the product key is entered! Let alone the validation successful screen.
Windows 8 does not permit installation without entering a product key, Windows 7 did. With Win7 the easiest work-around is to install Win7 without entering a product key. Then before doing anything else, install it again. Right over itself.
So to me the question of the Windows 8 clean install of the upgrade version remains unanswered. My plan, which I did test, is to do a quick Win7 install with no product key entered, then install Windows 8. This does work and only takes an extra 20 or so minutes.
One big caveat follows: Windows 7 drivers are not necessarily replaced when Windows 8 installs and Windows 8 does not necessarily work with Windows 7 drivers. So you will get an upgrade installation that needs some tweaking. As in reinstall the hardware drivers that come with your motherboard after downloading the Windows 8 versions.
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