The alphabet soup are fare buckets. J/C are business class, YMUABVQL blah blah are all economy (generally ordered from most expensive least restrictive to least expensive but most restrictive, except for that last H fare... H goes somewhere between B and V... Air Canada spelling, not mine)

J8 C5 Y9 M7 U7 A7 B3 V3 Q3 L0 R3 I0 N0 G0 P0 E0 T0 H3

The number means how many seats left in the "bucket" that AC is willing to sell. So L0 means that if you want to buy a "fun" fare, the lowest one they will sell you in that bucket is a "Q" fare for more money (since Q still says they are willing to release 3 more seats). Note that as they release seats in the lower fares, the plane is filling up and seats in the higher fare buckets may also decrease as well... so if you purchase an R fare on this flight, you may see B,H,V, and Q all drop to 2 available (because you just took one of the lowest seats available out of inventory.

Keep in mind too that when you see a 9, it doesn't mean only 9 seats are left in the fare bucket... the system is merely telling you that AT LEAST 9 seats are being offered. So seeing J9 C9 Y9 M9 U9 B9 V9 Q9 L9 R9 I9 G9 N9 P9 etc. usually means that the flight is pretty darned empty!

As for upgrading, watch that C fare bucket. As long as it shows even ONE seat available in that bucket at your four day mark... call AC ticketing to reserve that upgrade. You'll have to do it again for your return SFO-YYZ-YUL flight... and remember that you only need the one SSWU for that return trip as long as you aren't doing a stopover in YYZ.

To clarify "waitlisting", if you don't get your coveted C fare upgrade, then get to the airport as early as you can to get on the waitlist there for an upgrade. You will get any unsold seats in J, as long as there aren't any other Super Elites, Reward ticket holders confirmed in economy but ticketed for business, or Elites who checked in earlier than you. Don't be resigned if you can't get a C upgrade in advance, just get yourself put on the AIRPORT waitlist as soon as you can.