I have found that the change itinery does not work if you are paying for the difference with TTF, just as you found. If you cancel that first flight and then book the new itinery as a new booking and use the old TTF and the new TTF, then it will work. Clearly a bug in the new system. As long as we are talking about bugs, if you book a new flight while logged in and pay with a credit card on file, then you do not need to enter any additional info. However, if you pay for the entire flight with TTF, then you must enter your 'billing info'. It is a little unfair to ask the CSA about the website. They know nothing about it and cannot help you with it.