How does the domestic change policy work where you don't want to re-book straight away onto another flight, but merely hold your credit to use toward a yet to be determined fare in the next 12 months. ie I want to cancel and then hold the residual balance for use at a later date.

Can you get a "credit" for future use and can you do this on line.

Further, if you can get such a "credit" can you merge two or more of them into a booking in the future and can this be done on line?