All you have to do is begin booking the sale fare. When you get to the point in the booking process where they are asking for your CC info stop and minimize the screen. Then open the SW site in another window, go to the Travel Center and cancel your old reservations. When you finish cancelling the old reservations it will give you a confirmation # (which is the same as the old confirmation # BTW)...write that down.
Then go back and reopen the screen where you were booking the new flight and finish booking. When you see the tab on the payment page that syas "Apply Ticketless Travel Funds" click that and enter the confirmation number you wrote down. Finish the booking process and voila! On the confirmation page for the new flights it will tell you how much you have remaining in credit in your Ticketless Travel Funds account.