I don't at all blame them for being paranoid, but I do want to know whether I'm going to be allowed to board my flight!

To address PVDtoDEL's comment, I've already asked for a refund from a different merchant (cleartrip) for a different charge to the old card - and so far it hasn't gone through. Hm, let's see who pockets the money! As far as I know, the card number is still valid for refunds, but not charges (at least, that has been my past experience with cancelled cards).