You should have no problem being switched to alternate flights within a day or two of the original itinerary, so long as there are actual PY flights to be switched to, and you present those to the agent in a clear and polite manner.

What date are you flying? What flights are you currently on? It's likely that there just aren't any YUL-YYZ flights with a PY cabin that make a legal connection (4 hours I believe, not sure if that can be overridden in this case) to your YYZ-LAX flight.

Going after a partial refund is likely not worth your time. 26k is a cheap enough deal that the fare difference between your two flights will be barely anything no matter how you calculate it.