Aircraft changes on this route happen all the time. I have lucked out and also been screwed by this. Sometimes the syd-yvr route is late...sometimes the plane goes mechanical enroute so they swap it out to fill the remaining route. YMMV! Usually EF has some pretty useful information of what's going on to help give you some clues.

here's what it says today...AC0034/19JAN EQU ACA77LB01
CTY TML ARR DEP GRND AIR CABINS J O Y
SYD - 1215 - 14.15
YVR 0730 0900 01.30 04.20
YYZ 1620 TOTAL TIME SYDYYZ 20.05
RMKS/DLY LATE INBNDDELAYED OFF LATE INBOUND AC034
SYD 1244 00:29L YVR 0803 00:33L 77L SOP AD AA
DEP TML GATE --/59
ARR TML M GATE --/-----
YVR 0936 00:36L YYZ 1654 00:34L 77L TRU AD AA
DEP TML M GATE --/C51
ARR TML T1 GATE --/-----