I had an upgrade confirmed at T-2 days on an upcoming YYC-YYZ (I was pleasantly surprised to get the upgrade - I'm batting about 10% on YYC-YYZ flights for eUpgrades), and had selected my seat online and printed out my itinerary with seat 2D on it. Went to do OLCI and now I'm in 3F and all the other seats in J are taken, so I can't get an aisle seat. Equipment is still the same (E90).

It looks to me like someone changed my seat assignment to give someone else the aisle seat I'd selected and moved me to a window seat. I thought once you selected your seat, you had it unless there was an equipment change or unusual event. Not the end of the world, as at least I'm in J, but I really prefer aisle seats, so am a bit annoyed that my seat assignment was changed. Does this happen often?