Your best bet when cancelling an existing ressie to book a Ding fare is to open two windows and work from both. Use the first window to go as far as you can for the Ding fare up to the screen where payment is required, then, go to travel center on your second window and scroll down to cancel and complete all of your info and cancel......now immediately go back to first window and fill in your confirmation # from the cancelled ressie in the box for ticketless travel funds. Just be aware you CAN lose your ressie if the Ding fare fills up before you can accomplish this. It seems like to takes forever to do, but, in reality it's probably a minute or so. I've done it many times with no problems......just did it this afternoon for six new ressies, using four different TTF confirmation #'s from previous flights.