Southwest will let you do it, in fact they will even help you.
When I was long distance dating from Florida to California either myself or my "wife to be" was flying cross country on average every 3 weeks for a year. In the post 9/11 efforts to keep their planes full WN
was offering Internet Specials and promotional fares constantly. And since we were flying so much we would have 3 to 4 outstanding reservations at any one time.
I used to watch the sales, and when they offered a better deal, I would rebook all of our reservations. I used the website for all of this. This is how if works:
if you find a lower fare than the one you have booked, you cancel the existing reservation, and rebook at the lower fare. The left over funds go into a pool called "Ticketless Travel Funds" That money is available for up to one year from when you 1st purchased that ticket to book any new flight you want, even the one you just cancelled (have to be careful not to loose the last cheap seat though)
I felt kinda dirty sometimes as I felt like I was kinda cheating them, but in the end it worked out. We got to fly more, and WN
ended up with 2 fiercely loyal customers. We will never fly another airline domestic, unless WN
doesn't offer service anywhere near where we are going. Of course, this policy doesn't help you if you are going to Hawaii or International.
And after she moved to FL
and our flying frequency went down I actually ended up with some funds I couldn't use before they expired. I called up and asked for an extension, or told them they could have the money as a thank you gesture. The agent would hear nothing of it and almost insisted he refund the amounts to my credit card. We aren't talking much money, but the gesture meant a whole lot.
Now unless you fly a whole lot the game is probably not worth it. But it got us through a rough time. Even if I was red eyed on the computer trying to keep track of all the reservation codes.
Thank God it's over.
Of course WN
also plays the different price for different times/seasons/time between purchase and actual flight games. But they lay if all out on the website.... Show you all available fares for your particular itinerary, depending on what options you want. And there are never more than 6-7 fare types.
The trick is to book if you think it's a good deal, see if you get lucky and catch an "Internet One Way" special, and rebook if it comes up. At least that gives you get a chance to reuse the money. Glad to head United will do a similar thing, that shows respect to the customer.