how would this work?
 

ok Ive got a flight currently booked going YVR - YUL thru YYZ on #142, it leaves at 815, there is a flight direct YVR - YUL at 8am. I am booked on a C class fare on #142, the other other flight that leaves just before it is showing C0, if when I am at the airport decide to switch to the earlier and more direct flight, will I have to upgrade to J class fare, or will I be able to get on with a C fare, assuming it is not J0, and how would it work with upgrades? like would I get priority or anything, i have no status btw.

You can stand by for C on AC150 when you check in and will likely get on. AC142 is a much nicer ride though if you have the time.

And you get more mileage with the additional segment worth 625 miles rather than the additional 100 or so +25% that you'd earn on the non-stop.

I don't think I will get the extra milage, because it is all one flight number, and yeah the main factor for taking #150 would be time, because if #142 is late then im screwed.

No additional mileage on direct flights. All you get is YVR/YUL + 25% CoS.

Several times I have gotten off of 142 in YYZ and taken an earlier flight to YUL. (I think there are two flights earlier than 142 leaves again). This gets you in earlier to YUL plus the extra miles.