Sure, when they chand your flight you can feel free to change your seat.
I didn't know their where planes with bassinets.
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American Airlines is notorious for changing flights, flight times and seats. They have done this to us many times. My wife sometimes get airsick and she needs a window seat. One time they changed our seats which were together and placed me on one end of the plane and her on the other. If the new assignment isn't good for you then by all means change it. Let them know.
Some models of Boeing 747, 757, 767 and 777 have 'bassinet' rows. They are rows of seats with a bassinet in front of them for people traveling with babies - usually long distance flights, i.e. international.
the reason I spend the extra money for choice extra is that I am 6 feet 2 inches tall and it is my first time in a plane. I was thinking about switching both flights the one to Miami to get in at 11:50 am instead of 1:10 pm and take a latter flight that leaves at 3 pm on the back.
Flying is complicated - just be sure to factor in the time it takes to get to the airport AND to get to your plane. Security can easily take 30 minutes - even an hour, although that would be a bit unusual.
You will even have to go through security twice if you are flying international, because you have to clear customs on your first landing in the US. In Europe, you never know but it is not uncommon that you have to go through security to change wings at the airport.
Always leave at least 90 minutes is my advice. I have made it on less (60 minutes) but it can be a matter of luck. I have also missed planes due to too short layover times.