24L is getting quite a workout on the AC 16 flight LOL. ( CX may run out of its complimentary drinks when I finally pay up with freebies). This is one of the routes I really know backwards and forwards so here goes;
- The YYZ-HKG route is one of the busiest and most likely to be full. This means that unless one is super duper AC Altitude status, it is unlikely that one will score an easy upgrade. (It's easier for AC altitude status holders if flying midweek.)
- I will be surprised if a United Gold gets anything other than disdain.
- It's a long flight and the food is as bad as United, so I caution you to consider eating something at the airport.

I don't know how you are connecting in HKG but I assume that you will use the CX option. CX will require a bit of a dash if the flight arrives late as usual. If it is TG, the gate is in close proximity to the AC arrival. If you run into a missed connection go immediately to the transfer counter. These people are awesome. I have never had such fantastic service anywhere else, after I arrived late on EVA and missed my TG flight only to be told by them to shush, go have a drink in the lounge while they collected my bag from arrivals and rechecked it on a TG flight for me. One of the best airports to have IROPS or a missed connection, because of their efficiency and politeness even when being screamed at by snotties. However a cautionary note: They seem to have more success with the premium Asian carriers than AC, probably because they have more authority to fix things up. AC doesn't seem to empower them the way BR/TG/SQ does.