I would call AA and explain to them that you had a seat and then had it taken away. See if they can give you one back. If you were unseated because of an a/c swap, they should be able to get you one back.

If for some reason you cannot get a seat assigned again now, that doesn't mean you will likely get bumped. I would try to check-in online exactly 24 hours before your flight. If it does tell you that you must do so at the airport, then just be sure to get to the airport a few hours before your flight. Even if you don't get a seat at check-in, the odds you will be bumped are low as long as you check-in a good amount of time prior to check-in cut off for the flight. They block off a lot of seats for these big planes for family seating, special assistance passengers, seats from people to pay extra for like exit rows main cabin extra etc. Those eventually get released closer to flight time.

But again, if you had a seat before, I would say your odds of calling and getting them to assign you one are pretty good. Be persistent and if the agent is unhelpful hang up can call again.