One thing to point out - even if it is a high back booster - if you are still using as a convertible carseat with the harness straps, yes it is okay.
HOWEVER, if you are using as a booster (where you put your car's seatbelt over your child's shoulder) then it is not okay.
My guess having a five year old who is about the same weight as yours is that you are doing the latter.
Last year, while my daughter still fit with the harness straps we used the seat on the plane. However, because she was already so big, she was very uncomfortable and was really too close to the seat in front of her. So now we don't use it anymore.
I haven't flown Continental for a few years, but I used to have a job that required me to fly every week, often on Continental. My only complaint with the airline is that they never had enough blankets and pillows and their planes were always freezing. Make sure you take sweatshirts for the plane and try to get blankets and pillows as soon as you get on if they haven't been left in your seat. That way if they run out of blankets and the plane gets cold, you won't be one of the people without one.