Is it possible to change an old reservation (use the residual value of an old reservation by finding it under "Manage Reservations" -> "Cancelled" and also use an e-certificate when booking a new flight? I don't see the option to "Redeem a Travel Certificate" when changing an old reservation. I'm afraid that if I hit the button that says change, it's going to start processing (and billing me for) the new reservation without giving me a chance to apply a travel certificate to it.