So a little insight before I offer my info on this post. I originally booked a roundtrip fare (no discount) when the fares went way down a few weeks ago. I called to change the trip and used the code (round trip again). After further investigating, I decided to cancel the return flight (since I chose the latest return date at the time..2 days before our actual return) and book later.
I asked the Customer service lady this. If I call and rebook my entire trip can I use the discount. She said yes! She explained that since I will be canceling my trip, no matter how it is credited (future travel or back to a credit card within 24 hours of booking) it will show as a "new booking" therefore I will be able to use the discount.
She also told me that it is only for a roundtrip ticket..not one way.
So..if I decide to change my outbound (which I am hoping to do) and cancel that..then turn around and book the total roundtrip fare I will be able to use the discount code on all of our tickets. I'm watching SWA like a hawk for their anniversary sale within the next few weeks. Hope that helped and wasn't too confusing.