in order to take advantage of the new Ding fare, you will need to first cancel your original reservation...this is what I usually do ~
1) pull up the Ding window and select your flights all the way up to the payment window
2) open a second window and go to travel center and select "cancel flight"
3) enter your original reservation confirmation number and name of person that paid with a cc
4) before you go ahead cancel the first reservation, go back to the Ding window and verify that all your seats are still available. Sometimes they can sell out while you are doing the other steps
5)if your Ding fares are still available, go back to the cancel window and cancel the first reservation
6) quickly go to the Ding window and where it says "pay with Ticketless Travel Funds", enter in the confirmation number and name of person that paid with a cc from the first reservation that you just cancelled. You will still need to enter the cc info and names of passengers, but nothing should be charged to your cc.
7) SW will apply the credit from the cancelled reservation and the difference will be available as ticketless travel funds for another trip.
HTH