I've always ran into problems changing shuttle flights. The rules used to say "No SDC on Shuttle" but whenever I called, they could eventually (after getting supervisors, etc.) change my flight. One time I called outside of the SDC window and they said, "Sorry we can't change it outside of the SDC window. Oh, wait, you have a transferrable fare, so sure what time do you want to move it?". The Shuttle rules need more clearly communicated to call staff.