Key points of AC COVID change/cancellation policy [as of December 10, 2021]:
(see this page under "Changes and cancellations")

Unlimited changes. No change fees. – Until December 31, 2021, if you want to change your flight, we'll waive the change fee. After December 31, 2021, you can make one change at no extra charge. If your new fare has a higher price, you only need to pay the difference from your original fare.

If Air Canada changes the time of your flight, you make a change for free.*
*Within 3 days for North American destinations and 7 days for international and sun destinations.

If your flight is cancelled, you get a refund – If your flight is cancelled for any reason and we don’t rebook you on another flight that departs or arrives within three hours of your original departure or arrival time, or if we add a connection to your itinerary, you can request a refund.

Need to cancel? Save the value for future travel – If you need to cancel a booking, the full value can be transferred to an Air Canada Travel Voucher, which never expires and is fully transferrable, or converted into Aeroplan points with a 65% bonus. Refundable tickets are always refundable.

Flexibility with Aeroplan bookings – Until December 31, 2021, if you want to change your Aeroplan flight reward, we'll waive all change fees. After December 31, 2021, you can still make one change without a fee.