Call SWA and rebook at the lower rate. The agent on the phone will help you or walk you through doing it online after they cancel your tickets you can use the credit yourself over the internet.
When I had to make flight changes the agent had me look up the fares while I was on the phone with her and see the price was still available, then she cancelled my old tickets in her computer, and told me to proceed through the booking process like I did when I initially booked them online and click the "use previous credit funds" tab or whatever, and then they were applied. Very easy!