AFAIK - it should be okay to keep the key since you purchased it to upgrade. Let's assume the scenario, you buy the key, power outage happens, you are in a blackout for a day or two. (It happens!). Or, say that you buy the key, and an emergency happens, you can't get back to installing later.
I would figure that Microsoft would allow for something like that. If in doubt, a quick call to them might also do the trick to be absolutely sure.