I have a ThinkPad W520 . It only holds one hard drive at a time. It has Windows 7 Pro that came installed on it. I also have a spare hard drive that also has Windows 7 Pro installed on it using the same key. If I execute the free upgrade from Windows 7 to Windows 10, can I later remove the upgraded drive and put the Windows 7 drive back in? Specifically, will my key be invalid on Windows 7 now? I will only use one of the drives at a time, but I became concerned when I read that my key will be 'converted' to a Windows 10 key.
I was in the exact same situation last week. Turns out the Windows 10 product is added to your key on the Microsoft systems.
You won't lose Windows 7 access by claiming your Win10 production upgrade entitlement.
That said, when you do want to go win10 in future with this device it will just work without needing to buy a licence key in future.