As this is not a voluntary change airlines in Canada are legally required to rebook on the next available flight. They cannot force you to take an earlier flight. They cannot request additional payment. The airline must even rebook you on another carrier if they can't get you out within nine hours. The only reason AC won't rebook you on a later flight might be that you're doing an overnight connection in YYZ. Then they are within their rights to require you to take an earlier connecting flight as your ticket is A to B, not A to B to C.