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If they are both Retail copies, the upgrade is supposedly a Retail Upgrade and can be moved, right?From a purely technical standpoint if you have a legit activated Windows 7 install and a legit activated Windows 8/8.1 install (which is NOT an upgrade from Windows 7) on the same machine then technically yes you're entitled to a Windows 10 upgrade for both OSes. The issue is them being on the same machine because the hardware hash generated by the Windows 10 upgrade is what's used for activation and you shouldn't be able to get two Windows 10 activations on the same hardware hash.
Only one way to find out, I suppose: try it and see what happens, but realize that once you do this you're still stuck using that same hardware, you can't transfer the license or activation to any other hardware so, you can't do the Windows 7 install to Windows 10 upgrade on that machine and then bump the 8/8.1 install to Windows 10 upgrade on another, it doesn't work that way.