I also heard (unofficially) that AC and CP would consolidate all domestic and USA flights to T2 and all other international flights to T3. I can't see them taking over T1 at all, especially considering their lounge facilities, and other stuff that's already set up at T3 from CP.
Just a couple of points to consider. First of all, I think YYZ is going to have quite a reduction in flights. Sure, it will get new routes to cities such as Munich, Amsterdam, and into the USA to Austin Texas and Orange County California, among others, but what about the traffic that's going to get moved out of YYZ? What about the traffic that will be taken out of YYZ from new routes such as Halifax-Vancouver, Edmonton-Montreal, or Calgary-Newark, or from plans to increase service on current routes such as Vancouver-Montreal and Winnipeg-Ottawa? Also, what about the capacity reduction to trim excess flights and extra seats from the network, at least in part on routes into YYZ (especially CP's triangle schedule). I think that although it will be bigger than AC was on its own, the YYZ hub will shrink from the combined separate operations of CP and AC in the past few years.
Also, what about the new terminal being built to replace T1 and T2? Sure, AC and CP will probably move into there together when it's done, leaving T3 for current tenants and those from T1, but that's some time away yet. In the meantime, I don't think AC really wants to use T1 if it can avoid it. If anything, it would just divert flights away from YYZ onto another routing so that it could fit everything into T2 (remember, they can stagger flights to use gates more of the time to squeeze more capacity out of it). I wonder if AC is going to suddenly be a supporter of the "T-NEW" project to replace T1 and T2 now? They were pretty opposed to it in the past as too expensive, but maybe now they figure they'll need it. Maybe if they can shutdown T1 earlier than they had hoped by a net reduction of operations by CP at T3, then they could speed up T-NEW construction? I've heard YYZ T3 is underutilized, how much free space is there? How many gates are at T1?