If you have passed the 24 hours window to change the Air Canada Booking, you need to pay a fee to change the flights. Generally, the airline will charge you a 100 USD fee for standard flights. But this is for flights that will depart within 60 or less days time. However, if the flights depart more than 60 days after the time of change, you only need to pay 25 USD. However, if you choose another fare or higher fare option, you will need to pay a fare difference.