When you bought the flight pass, it listed the airports the pass was valid for. They removed an airport from the list, it's now a breach of the original T&C you AND AC agreed to. It's the same argument CanadianCow made regarding flights/routes vs destinations.

I have had success in the past with AC whenever I pointed out the T&C and how I was due compensation for each instance. Their first response is usually to deny compensation, but if you can point out where the holes are in the T&C they will issue proper compensation. But remember to be diplomatic and leave emotions out of your correspondences.