this has worked for me on YVR to YYZ. on Sunday's after long weekends. could just be a fluke.

if you read the tarriff, check in almost 24 hours out insist that you will be getting on the flight come hell or high water and know that the flight is very oversold, AC will work with you, esp. if they know the three flights before and after, esp. after a long weekend, are also jammed. they do not want to explain to Mr. Lat fare or Mr. T+ fare that sorry, Mr. T fare trumps your fare even though you paid more since the tarriff is pretty clear, special needs, then J then Y and the rest are first checked in, first on (regardless of your little fare code).

also, if you are on the last flight from YVR to YYZ that arrives at the last reasonable time in YYZ for people to go to work on Monday or Tuesday after said long weekend when you know flights before are so jammed, you can make it clear you will demand the tarriff is enforced if this is oversold, despite AC thinking they can jam you with your cheap T ticket by purporting to suggest they can not give you an assigned seat even though 90% of pax have not even checked in (other than J and Y that trump us all even in the first come first served tarriff). since you are checked in so early, you know this.

you can advise them you will gladly go on any flight earlier or later, even on the day you check in, Sat...if, and only if, they put you in J.

worked two times after getting the shaft the first.

i think it only works if many flights that day are oversold and you have a cheap ticket and only if you check in really early.

note, in both cases, even 24 hours out, i have been given a "Gate" bording card and AC has purported that even though i have been 24 hours out they could not assign a seat...cough.....Bu__$hit.

good luck. as i noted, could just be a fluke.