I wanted to comment on this as I had to go through the change process just this morning when my $79 flight dropped to $49. I always thought you had to cancel and rebook as well....but after talking to SWA this morning about it (had to call for a different reason), the CSA told me I could just change the reservation online!
Go through the Travel Center, then select "change airline reservation". I was prompted for my confirmation number, where and when I wanted to fly, and then they gave me all the available options for that date. I selected the flight I wanted at the discount price, and they applied funds from my previously-paid flight and gave me a $60 credit! VOILA! Painless and no fee!! I was careful to read everything to be sure there would be no fees associated with this change, and there weren't any!
So after all was said and done, we got RT for 2...nonstop from Manchester, NH to MCO for $230!
I really do love SWA!