If you opt for the upgrade you get an activation key in the e-mail. After that you can (and should) do a clean install.
That's what I was thinking about as a possibility. At least the Win7 upgrade versions sold on optical disks include an activation key.
The one I have installed is "designed for Windows Vista" which I've never used in my computer and includes both 32 and 64 bits disks plus one activation key. I wonder what will happen the fourth time the key gets used, it's been installed on two machines so far.
As for a modern Windows 8 computer, doesn't the activation key get saved somewhere in the UEFI? At least that's the case in some brand computers, in case of a hard disk replacement the reinstall can find the activation key from within the computer, allowing a clean install. In such a case there'd be no problem to prove the existence of a previous version.