You need to find a UA supervisor willing to conference in AC to get you on NRT-YVR-YUL.
This was an AC schedule change, AC should be able to do it. Your biggest challenge is finding someone over at UA who knows how to get AC to do this.
Call UA and ask for the Alliance Desk.
Assuming this is Y, NRT-YVR is available, but YVR-YUL is the one that needs forcing.