FYI. In May, I wanted to change our ressies from a Click n save fare to a Ding fare. You cannot do that on the "Change your flight" screen. The Ding flight must be a whole new reservation. A little gymnastics and I saved about $100.
How I got around it so I didn't lose my original flights and using the same money I had already paid: I called customer service and at the same time I was booking online the new Ding flights. When it comes up with payment, I had to wait for the customer service rep to cancel our flights and the money became immediately available. According to my friend who happens to be a CSR at SW, sometimes the money isn't available right away. It might take a minute to an hour for it to become available if I had cancelled online and then try to rebook. I got a credit towards future flights which I am going to use soon. It took a few minutes to do this. The CSR was very nice and worked great.