Many airlines will give you a voucher, good for travel on a flight within the next year, in the amount of the fare difference.

Only thing is, some airlines charge up to $100 (more for international tickets) in an "administrative" fee to reissue your ticket at the lower fare. But the good news is that some don't. Last we checked, United, JetBlue, and Southwest will issue a credit voucher without deducting a fee. Continental, Delta, and American charge the usual fees, anywhere from $50 to $200 depending on the destination. Northwest charges just $25 for both domestic and international fares.

But even those airlines whose official policy is to deduct a fee sometimes waive it. So if you don't like the first answer you get, try again and perhaps you'll get the full amount.