In my expierence Microsoft is pretty lienient with activating products, I would expect a little moreso with an upgrade. My guess would be that if you reinstalled it on one of the computers (or on a third) it may make you call them. Calling them is really simple, you read the numbers off the screen, and if it doesn't like you, then you talk to a person who asks "is this installed on any other computers" You answer, they give you the key.
I wouldn't worry about it too much, Microsoft isn't going to remotely detonate your computer or anything.
