In my experience none of the previous responses are quite right with respect to AC's cancellation policy. Don't worry about B1's concern; AFAIK all AC non-refundable fares can be cancelled (subject to change fee). I think there was a time where in the "a-la-carte" pricing scheme that you could take an additional price deduction in exchange for "no changes" but I don't think they do this any more (I think this was only available on Tango fares).
If you want to cancel a flight, phone AC. You do not need to change to a different flight, the residual value will stay on file as a credit. The change fees will be due when you go to book your changed flights.
calligow, to answer your question, if you have a $400 flight and you cancel, you will have a $400 flight credit. The change fee will be paid in addition to this, no matter what.
For all of the following examples assume that you have a $400 flight that you cancel (Tango, non-refundable on a domestic itinerary) and the change fee is $150 (because there were 2 directions of travel prevously cancelled):
- If your new flight is worth $400, there will be no fare difference and you will have to pay the $150 change fee.
- If your new flight is worth $450, there will be a fare difference of $50 and you will have to pay the $150 change fee.
- If your new flight is only worth $100, you will forfeit $300 worth of ticket value, and you will still have to pay the $150 change fee. THE RESIDUAL VALUE OF YOUR ORIGINAL TICKET CANNOT BE APPLIED TOWARDS CHANGE FEES.
My advice is that if you ever have to cancel a trip with AC, avoid rebooking the lowest fare and rebook into a fare class that uses up as much of the original ticket value as possible. This way if you have to cancel, or change, again the value of your ticket that carries forward will be larger. Using the example above, if you have to cancel your ticket again, and you had re-booked a lower cost ticket, AC's system would show that you only have a $100 flight credit instead of $400, as that was the value of the most recently cancelled ticket.