YYZ-HKG is quite light loading i believe. I flew on 829 recently, in the latter part of the plane, there were more than 30 empty seats at the back of the plane, considering the only last few rows in the plane. and, there were quite a lot of empty seats all around the plane, i.e. those B/E/J seats. so, i guess rebooking them on AC or put them on 829 should not be too difficult to ease the pax in yyz.