You will pay fare difference plus the change fee per change 'event', so figure out all your changes and do them all at once. Otherwise, you will end up paying the change fee more than once. And AA subtracts the change fee from the current value of your ticket and applies the residual to your new fare. If you luck out and the new fare + change fee is actually less than the value of the existing ticket, you will receive a voucher. I think it will be a paper voucher, meaning when you apply the voucher you will have to contact AA and mail the voucher back to them.