Southwest will credit the difference to your "account". That is, you can use the difference, to purchase another ticket, up to one year from the original purchase date. You need to keep your confirmation number, as the funds are allocated by that number to your name. I have two flights recently on which the price dropped about $40 each, so I have $80 to use on a future WN flight. I think they let you combine up to five confirmation numbers to make one purchase, but I am not sure. Someone more familiar with the ins and outs of this policy can likely chime in with an update.

I believe WN to be the most liberal in this area. It would shock me to find out that any of the "Big 6" offer anything like this, except, perhaps, to their highest elites.

Regards,
Stevekoe