What they will have to do at YYZ is provide a sterile shuttle to T2, feeding into a sterile US pre-inspection station. This will require some construction, as they cannot use caroussel 99 for this.

They will also need to run the shuttle the other way for US-International.

YUL is doable in the US->Int direction with the construction of a simple stairway+elevator from below, and a segregated channel for US PFI.

In the other direction, transit pax will simply take the moving sidewalk to the Aéroquay. However, this means that the AC flights have to move from Gates 1-5, sing that area and the MLL are not sterile. This will entail the construction of a lounge facility.

YHZ won't work, because there is nowhere to keep International pax segregated.