Below is the letter that we sent to Air Canada...they have not replied and I don't think they will...by the way, we've called but they wouldn't put us through to anyone of importance to discuss the issue:
2 tickets for London are $2401.76 from April 12-May 6. We have a credit of $2300.00 and we are aware that we have to pay $80.00 per person for changing reservation as a penalty and $100.00 for both of us as a difference in price, which is a total of $260.00. Your agents are saying that we have to pay additional $600.00 per person, which is half of the price for airfare, to compensate for fuel surcharges and some other charges. My way of thinking is, if you have $2300.00 of our money as a credit, why would you keep $470.00 of our money per person (where did that money disappear?) and charge us additional $600.00 per person, calling it fuel surcharges nowadays and you didn't have it before, when we canceled trip to Mexico in July. If it would help, I can send you screen shots of our canceled ticket with the price break down and the screen shot of the fair that we wish to book.
What next? Good luck to us all.