FWIW, as part of trying to get some crap Sony laptop drivers working with Windows 10 I installed a clean copy of Windows 8.0, didn't install any updates, then upgraded to Windows 10 via an iso loaded on a USB stick. I didn't even have it connected to a network until well after Windows 10 was installed.

I had already performed the Windows 10 upgrade once on the computer, so it was registered. I would guess if you're doing it for the first time you may have to activate the copy of Win 7/8 first, but maybe just having a valid product key is enough?