Currently this is the only city pair where Eastbound is odd # and Westbound is even #. (Because AC designates this as a TransPac flight) AC wants to change that to match all the other flights so Eastbound even/Westbound odd and It'll start on the winter schedule. They can't just reverse AC051 & AC052 on Oct 31 because you run into the possibility of having the same flight number (AC051) on the same calendar day should YYZ-DEL delays and doesn't arrive until Oct 31. So it's logical to use a new set of flight numbers AC054/055.