Well YYZ has a crew base unlike JFK. Since it was 8 hours total flight time, the orginal crew would have exceeded the maximum hours of operating time. Most likely, a spare YYZ crew was called in to continue the flight. Also, an aircraft was available which was scheduled to do YYZ-HKG. So the a/c that was scheduled to do YYZ-HKG, was used for the continuation of JFK-HKG and the and the problem a/c was used for YYZ-HKG which they delayed. Cheers