United will give you a voucher good for one year for the fare difference. However, my experience has been that the voucher is hard to use. No way to use it electronically, meaning you either have to take it to a ticket counter or to a travel agent (the latter requiring a fee to the travel agent).

Southwest, on the other hand, will give you ticketless funds, also good for one year. No change fees, no penalties. Just cancel your original flight and rebook at the new price. Your savings stays in the "Southwest bank" under your original PNR number. Fly again with Southwest and use ALL of your money.

Granted, Southwest doesn't go to Mexico yet, but maybe soon?!? We can only hope! Southwest does go to Hawaii using connections with ATA.