So one of the legs of my upcoming trip has had an equipment change. It has gone from an A319 (mainline) to an A320 on AC1140 YUL-YHZ. My seat assignments are still the same.

So here's my question. If I had been booked in bog standard row 18 on the A319, would I have stayed in it on the A320 where it has the best preferred seats on the plane according to seatguru? Or would I have been forced to select new seats that aren't preferred.

If I would have gotten the free upgrade, do people choose row 18 on the A319 just on the chance that it gets upsized to an A320. If I move to row 29, will it give me free extra room if it gets upsized again to an A321.