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Yes 1-2-1 means you'll have the new J "BEST" seat ^ and YVR was indeed among the list of destinations concerned in the last update we had from AF.

I would recommend you to take a seat in the front cabin, because you will be served among 1st for the meals. The window seat is not giving more privacy compared to the middle seats, but the window seats pre-selection is reserved to Elite plus pax (or J/C/D fares, so not for award bookings or Z and I fares). So unless you are FB Elite+ or booked in one of the eligible booking class, you won't be able to pre-book a window seat (you'll be able to have one at OLCI if there are some remaining).

Risk of aircraft switch : yes it is always possible, as on all airlines. Impossible to predict. AF may also be late in aircraft retrofitting. At that stage, there's nothing you can do about this, so just relax

Yes, AF has an interline agreement with AC. But AF won't give you the BP for the AC flight (you will have to obtain it at a transfer desk).