You pay nothing when you cancel.
The value of the ticket can be used within 1 year (IIRC, the 1 year starts from the original ticket issue date).
Once you call in to book your new itinerary, the value of the old ticket will be applied to the new fare. Any change fees must be paid with 'new money' at the time the change is confirmed. That is, if you value of your old ticket is $1000 and the new one costs $900, you'll still have to pay $50 extra. Any difference in fare value in your favour is forfeited, and cannot be used to pay for the change fees.