You can go on-line to Southwest's website anytime (starting at 12:01 AM) the day of your flight and check-in. SW doesn't do seat assignments, not even at the gate, just assigns people to board in groups. First group to board are the "A" ticket holders, then the "B" and last the "C". Because we had "A", knew we woud be able to sit together, so we didn't even wait in line with the other A holders. Just sat comfortably in the gate area until pre-boarders got on, then the kids, DH, and I had only a moment to wait before we got on. You choose your seats oncee you are on the plane.
Sitting together may not be an issue if you are only a twosome or traveling without children. Or if your child is 4 or under, you can pre-board (so folks like us with 7 year olds really need the As or Bs to stay together).
If you have bags to check, you do have to wait in line for that, but your letter assignment is already set. In our case, we don't have a printer at home. During the baggage check-in, the SW person printed out our boarding passes (still got to keep the "A" assignment).
We changed our reservation and did not incur any fee. Am pretty sure that unlike other airlines, they don't charge fees (other than the flight fare change, if there's any). Good luck.