I just connected from the US to a Domestic flight in YUL today.

The most significant change is that the staff scanning BPs after CBSA check will either direct you through the glass doors to international gates or down a semi-finished hallway that spits you back on the Domestic side, close to the intersection between International, TB and Dom areas (by the YUL Pizza counter).

Very easy and smooth but it seems like you have to show your BP every 10 feet.

The new BP check for the international wing was also in full force with extra staff to help. It heard one agent say that once you go through to the International gates, you are not allowed back to the Domestic side. Gate 51 also now appears to be a hybrid gate with a glass partition that can be accessed either from the International side or the Domestic side.

Again, as it has been discussed upthread, OSS in YUL is so much smoother and efficient than YYZ because of the presence of CBSA automated kiosks, Nexus machines and no need to take a bus that leaves at intermittent intervals.