If you can find the rate/connection you want as two one-ways but they do not show if you try to book as an open jaw:

1) use the code for the city combos you find, online at aircanada.com

2) call AC reservations dept to make a free change to a different connecting flight with same fare class code . "since you are within the 24hr after purchase free refund period". (note that every agent I have spoken to seem happy doing this ...a refund/rebuy is much more work for them.)