To change a flight, you will have to cancel your complete itinerary and rebook. If you simply want to cancel and not rebook, you will be able to make that cancellation. You will not lose your commuter book vouchers when making a cancellation.
Regular, Ticketed Reservations:
To change a flight, you will have the option to change one or both segments of your trip. If selecting a new flight at a higher fare, you will be asked to provide a form of payment for the difference. If opting for a lower priced or equal-value flight and you are outside of the 24-hour booking window, you will be able to make the exchange. Refunds are not issued for the residual value of a lower-priced fare. If you need to cancel without rebooking, refunds are only issued if you cancel within 24 hours of booking or if you purchased a Refundable Fare.