For YYZ, YVR, and YUL, you shouldn't need to go through another full blown security doing a domestic AC to int'l. All 3 have someone checking boarding passes, etc... on the border between domestic int'l departures area but none , at least so far, force you to do the whole x-ray thing. You do face the screening if you go the other way.

YVR and YUL do have some gates (I think YUL only has one but c/b wrong) that can be both int'l and domestic but they are designed to be blocked into one zone or the other as needed.