I've done this with no problem. First, I made sure the flight times were available with the Ding fare. Then, I went into the "Travel Center", cancelled my old flight, noted my "Ticketless Travel Funds" # (or if you booked at full fare, you could just request a refund), and then went back through the Ding link to book the airfare. When you get to the payment page, you click the button that says "View Ticketless Travel Funds", enter the # you got from your cancellation, select that funds # from the next page, and then the fare is deducated from that amount. If you have any funds left over, they will be available for later use, up to a year from your booking date.