Originally Posted by jsloan

I would do it online unless you hav a specific reason not to.

The easiest way to do this is to (a) cancel the old ticket online; (b) begin a new reservation as if it were a brand new ticket; (c) when you get to the payment screen, you should be able to apply your FFC.

If this doesn't work, and it tells you that you need to make the change from within the old reservation, things get more complicated. However, this process, as written, should work 90%+ of the time if you haven't flown any segments on the original ticket. (If you have, this goes out the window).