Once you've upgraded you can extract the Win 10 key and use directly on a clean install or MS have said that it will detect the PC as already had Win 10 and not ask for the key at all. Personally I have grabbed the key from mine just in case.

I appreciate that. But I would expect Microsoft's systems to detect if you try to reuse an upgraded Windows 7 key. As the only valid reuse would be to downgrade to Windows 7 I would expect the associated Windows 10 key to be invalidated. It should be trivial to do this.