I believe we were all misled a bit by Microsoft when we thought we could use our genuine windows 7/8/8.1 key to do a clean install but it cant be done as far as I can understand. So biting the bullet I began the process of reinstalling 8.1 pro and activating then updating to win 10 pro which activated automatically as you describe, then I used the media creator tool to create the win 10 iso which I then used rufus to make a bootable uefi USB and performed a clean install of windows 10 ( I almost couldn't look at the activation area of system properties for fear that it hadn,t worked) but it was fine.
I tried for 6 days to activate a clean install of windows 10 using my OEM key extracted from the bios + 4 long sessions to Microsoft answer desk but as things began to unfold and checking out various posts around the net it became clear that you MUST do an upgrade on an activated win 7 /8 /8.1 system. you said this hash is based on hardware, if for example, I upgrade/ downgrade my RAM/ Processor or something as such, what happens then? Since it is all stored on Microsoft’s servers there is no reason for us to keep a backup either." From this point forward any future installs, including one where you delete all partitions and install Windows 10 from scratch, will be activated because of that unique hardware hash and the corresponding certificate. That same hardware hash is sent to Microsoft servers and a corresponding certificate is created to validate your systems activation status. This hardware hash is generated even if you choose to install Windows 10 with a Local Account. Since it is anonymous, you do not have to use a Microsoft Account. "The basic process is that when you perform an upgrade to Windows 10 (over a genuine Windows 7 or 8.1 system), an anonymous and unique hardware hash is generated that is based on your systems hardware configuration.
Once again, this should be a "sticky" :) No keys are involved