Most (but not all) tickets are changeable. You pay the difference in ticket price plus a change fee. If the new ticket is cheaper than the old one, you do get a refund.
The reason why the ticket is cheaper was that Air Canada opened up a lower fare class that previously was blocked. You can try to change your existing ticket to the new fare class. Expect to pay about $200 in change fees, but you may be be able get around $500 back this way.
You should read the confirmation e-mail that spells out the fare rules for your ticket. Make sure you understand what the rules and fees for changes are and whether the cheaper ticket comes with some extra fees (seat reservation, bags, etc.). Then call the airline directly.
If possible, make sure you know the fare class of your current ticket and the new fare class. Ideally you open the conversation with something like "Hi, can I change my ticket from a G fare to an A fare". If you don't know how to do get the fare classes, try "Hi, I see that the price on your website for my ticket is a lot lower now, can I change my ticket to the lower fare?"
You may be get lucky with an agent since up- or down-faring is about the smallest change you can do to a flight and they may waive or discount the change fee. This would also depend if you have any status with the airline.
Depending on the arrangement between Expedia and Air Canada, Air Canada may refuse to execute a change. In this case, you can try the same approach with Expedia, but I would start with the airline.
From Air Canada's website: https://www.aircanada.com/ca/en/aco/home/fly/customer-support/cancel-booking.html
My fare is not refundable: If you purchased a non refundable fare and wish to cancel your booking online, the value of your ticket will be banked for use as a future travel credit. To use the value of an unused ticket for future travel, please contact Air Canada Reservations. You may also want to refer to the Refund Services page for detailed information on how refunds are processed, and how you can apply your unused ticket value to the purchase of a new ticket.
Make sure you can quote this during the call with airline. If push comes to shove, you can cancel, and use the the travel credit to book to the new ticket. In this case you would not end up with cash but travel credit, but that's still better than nothing.