I recently made a last minute change to SW flights, it wasn't Orlando, but still the same idea.
I just want to point out that I looked online to see if the flights I wanted were available. They were and there was a "web only" price available which would cost us $40 pp more. I decided to call to see if there was any chance of changing the the flights with a lower or no cost (it was due to a funeral, so I thought it wouldn't hurt to ask). At any rate, to complete the transaction on the phone, it was going to cost $50 per person more because the web only price was a little better. The SW rep was very nice and told me that I would be better off doing online and proceded to tell me exactly how to do it.
The bottom line is, you can do it on their website or on the phone, but you may get a better price if you do it online.
Good luck!