I think you still might get a chance at a clean install, I would guess you start the upgrade using either usb flash 2.0 drive or dvd/cd drive from with-in windows. It will check for a valid key for windows 7, 8.1 if it can't find one, it will ask you for one. Then you can choose whether or not to do a clean install.
I actually installed an old preview version of windows 10 by upgrading from windows 7 without using the internet. Then I removed it, so I could go back to windows 7 legally. The upgrade was done in about an hour and the only issue was the desktop themes changed and some hardware but they would of been found if was connected to the internet.
One of the many issue I have, my system won't be able to play dvds
OEM keys were located in the bios on all installs.
I just though of something, a little off topic. Even though I have some privacy & cloud concerns with Cortana. If Cortana can do all that, I won't need to look at a computer much after the initial set-up. Cortana could read the webpages for me. I hope they allow Cortana to work with 3rd party browsers and programs as well. Instead of reading this forums, I could hear these forums. This would be excellent for blind users. I am not blind though.