How to change the aspect ratio during the film
Putting black bars on the top and the bottom of your frame looks like you changed your aspect-ratio, but really you didn't. All you did was put black bars on top of a 16x9 video. One way of doing this is to record in 2.35:1 and slowly zoom into your footage until the left and right part is cut out. Alternatively, you can just put black bars on your footage but let me tell you from personal experience: any half-way trained eye will look at it and say yuck.