a while back someone wrote about how they did it, and I just finished rebooking my flights to WDW at the end of aug. on SW-
if you go to SW home page, under "travel center" you will see a link for "cancel existing reservation" -complete all of it until you have to hit the cancel button-THEN, open a new window, make the new reservation with the new prices up to the point where you have to hit the reserve button- at that point, go back to the cancel screen and finish the transaction, then immediately go to the new booking and then, to get the credit from the cancelled flight, put in the confirmation number of the flight you just cancelled in that area of the new reservation and voila- it will credit you the $ from the first reservation-
as the other person explained, this way you dont' have to worry about cancelling the first one just to then find out that SW doesn't have the flights you want-
althoough, theoretically i guess that in those seconds between transactions, the flight availablity status could still change, but at least this way, you reduce the chances of it happening-
happy savings!
cathym