You need to check the website periodically (I check it almost daily). Just plug in your airports and dates and it will give you the flights and fares available. If your flight is available for a lower price than you paid, you can change it online. I was afraid to do it but it's really very simple.
You go into Travel Center, then select (Change flights/ itineraries, something like that). It will ask for your confirmation # and the credit card holder's name. Put that in and it will bring up the flights you have booked. Next to each one is a box, you select the one you want to change (or both if you are doing that), then it will bring up the flights and rates available. You just select the new rates and click Continue. It will show you the changes you are about to make, and the total cost. You have to click (I approve/ accept or something) and then AND ONLY THEN will the change take place. If you should happen to start the process and the flights become unavailable mid-transaction, don't worry. Your original flights/ rates are not cancelled until you click to accept the change. Basically the website walks you through it, and you can click HELP and then Changing Itinerary (or something) and it will basically tell you the same steps that I just did.
You may be able to make the change and get a credit over the phone, but if your new fare is an Internet special, then it is not available to the telephone agents (or so I was told). Then, you would HAVE to do it online. If the fares are not Internet but Promotional or something else AND you are not changing the flights, just the fares, you may be able to get a refund to your credit card. Otherwise it is a SW credit.