I just changed my tickets this morning for our tickets from Albany to Orlando. I tried to change the tickets without cancelling but the only fares that came up were the $125 and $187 (and we previously booked at a DING fare for $110, I think) so I called Southwest. The woman I spoke with told me (this only applies for DING, I believe) you need to cancel your current tickets and then book the cheaper price through the DING popup. She cancelled the tickets for me while I was on the phone with her and on the computer buying our new tickets for $88.
If the tickets you find aren't DING, I believe you don't have to cancel, just change them.
I know there are plenty of people on these boards who know a lot more about this than me but I just wanted to share my experience. We were only able to buy our tickets to Orlando because we're returning in November, so the credit we received ($75!!) can be used toward our return ticket.
Good luck