The key from windows 7 is no longer valid for 7 once you upgrade and keep 10 for a time. I think... On the up side, there are places such as Kinguin.com where you can pick up keys relatively on the cheap. Heads up though, sites like that are considered "grey market" and the key you're buying may not be genuine, although they do sell genuine keys and do their best to weed out the "black market" keys, it still happens, on the other hand, they sell warranties on their keys, so if you buy a warranty with your key, you're not necessarily out of luck, no warranty though, tough luck if microsoft bans that key. Also, even with the warranty, you could still potentially get key after key with no luck being genuine, course, the first one could be genuine. There's really no way to know what you're getting.