Booking 1 way on SW as suggested above is an excellent suggestion. When I go to rebook on DING fares I just start the booking via the DING link, when I get to the payment page I minimize the screen, re-enter the SW web site, go to the travel center and cancel the first set of flights, then as soon as I have the confirmation number for the cancellation (which is the same as the confirmation # was), I go back to my first screen and complete the DING fare booking by clicking on the "Apply Ticketless Travel Funds" tab.
I do it this way because you make sure your seats are available at the DING fare before you cancel your old reservation and by minimzing the DING booking it holds the seats for you while you cancel the other flight. I learned this the hard way...once cancelled our flights and when I went to book the DING fare not enough seats were available on the flight we wanted and we got stuck.