Don't wait for an OP-UP, use your e-credits.
Flight is oversold and this flight usually has a lot of E/SE travelers.

Upgrades will be given to LMU, e-upgrades, and then if there are still J seats left, to people who don't have a seat in the back (lucky OP-UP) .

AC8/4AUG looks much better than AC16 in J:

• AC16: J9C6D3Z0R0
• AC8: J9C9D9Z9R2

You can do a same-day change in HK, but the same class of reservation must be available on the other flight, which is unlikely unless you bought a Y/B/M expensive fare.
Right now, U class is the lowest available class on both AC16 and AC8.

It will be hard to secure the upgrade on AC16, and hard to change flight to AC8 on day of departure. If your class of reservation is U or above, you could change it now (assuming YVR-YYZ is also available)...

Good luck! ^