bonkas: The machines running Windows 10 have no license key.. I wiped the drives and installed Win10, no license required.. The only identifiable information microsoft could have to differentiate them is my logon and hardware hash of the PC.
Surely I don't have to wipe windows 10, Install 7, wait for the upgrade to become available then upgrade back to Win10.. Although I wouldnt put it past Microsoft to have to do this :)
Your license key is stored in BIOS