SW doesn't guarantee that the refund is automatic; however, I've done it many times and it always has been automatic. Tonight, I just cancelled and rebooked and then changed my mind about the flight time I chose and then cancelled and rebooked again. And used two refund codes as well. And everything was a-okay....
Best of luck.
Let us know.
A good piece of advice someone once gave me with SW which I used this time out is .... book your original departure and returns separately b/c to change when a price is lowered on a sale is a breeze online (b/c you just press change under "travel center" and check which flights you are changing) but when a DING comes it is easier to have your flights separated in case you only can use the DING on one part...saves a step.