On the origin if all AC flights are confirmed in Y, I have never been able to SDC to any J seat.
On the return flight, if you go to YVR early you it is likely you will be able to confirm SDC for the fee to a nonstop (but still a gray area I believe). If your route was ACJ cxn ACY it's a much easier sell to change to a nonstop ACJ but I have had some luck doing what you say.
Keep in mind that by SDC to the late yvr-yyz you are giving up full AC signature meal service in favor of the very abridged french omelette or the english omelette service...