It goes
YYZ-SYD, then YYZ-YVR and finally YVR-SYD
in that order
and of course, if YYZ-SYD is J0 and YYZ-YVR is J9, if you are wait-listed the whole way, they will not clear you in YYZ. If the segments are broken up, they will.
Of course, not everyone boarding in YYZ upgrading to J will go onward to SYD. So it may be J5 (YYZ) and J6 (YVR) and when 33 leaves YYZ, it becomes J2 (YYZ) and J3 (YVR)
You get the idea..
As a rule of thumb, your best chance in getting the upgrade is starting in YYZ...but they do release more Rs from YVR for this same reason.
This route AC tries to balance it as it is high demand and two major hubs.