It's worth noting that your Windows 7/8 installation MUST be activated correctly before upgrading to 10. (You don't need to have updates done) Also, you need to do an in-place upgrade from 7/8 to 10, then it should activate normally. Once 10 is activated, you can do a clean install if you wish. You do not ever specify a product key for 10 installs.
I had this problem on one of my upgrades. Had to reinstall 8.1, activate with BIOS key. Waited a little while, then upgraded to 10 again (in place). Didn't enter a key in 10 setup, then it activated normally.
Except it's nothing like that, because the Win 8 keys are not being overwritten with the Win 10 ones...
Correct. But so far I've only been able to do 3 computers this way... No luck today - getting blocked product key. "Apparently" the activation servers are overloaded.