I just went through this but havent taken the new flight. I had a trip to Ottowa that I had to cancel and wanted to use the credit for a flight to LAX. I found no way to do this on CO.com so I booked the flight online thinking they could just pick it up and apply the credit. Could not. They had to change my cancelled reservation under the original conf # to the new flight. I then cancelled my LAX reservation within the new 24hr window. Of course they used airline math on the $$$ and charged me the $100 change fee separatly and couldnt just subtract it from the value of the cancelled itinerary. They had to send me a $70 credit voucher in the end. I guess it works for them as they know I'll come back and use it but the whole process is confusing and not very customer friendly.