Ditto here, but according to Ghacks.com, who could be wrong, a key from a free upgraded OS is only good for that machine.
Since MS says your upgrade is free "for the life of the PC", it stands to reason that the key is machine specific.
"Once the upgrade process completes -- you will notice that when the system boots the Windows 10 desktop -- you have converted the product key successfully to a Windows 10 product key. This means that you can now use clean installation media to install the operating system. Note that it seems only possible to install Windows 10 anew on the PC you ran the upgrade on."
I only know what I read. I'd be interested to know where you got your info, please.