My ways to change video aspect ratio
At Playcus, we test out video commercials. One of the options of such tests is the aspect ratio of a video. It is difficult to specify clearly which format will work in which case. Besides, different platforms require both vertical and landscape videos depending where they will be shown. In order to find out which format works better we need to work with as similar videos as possible. The only thing that might differ is form factor. Therefore, we need to create videos in several formats right away.

The main formats we work with are 1:1, 9:16 and 16:9. For Facebook and Instagram news feed we mostly use square and vertical videos. Landscape videos are better suited for videos at Google Play. This is the current situation in our case. Some changes are likely to happen over time.
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Simple Background
Simple background is best suited for adaptation of very short and simple videos. In this case, homogenous background (color, gradient) fills out the whole area of the format, surrounding an animation enlarged to maximum extent.

Filling
When I work with this effect, I take square format as a basis. Then, I fit it into other format and fill empty spaces with certain effects (blur, glow, etc.) or nameplates with marketing materials (logotype, platform stickers, text or appeal).

This is a quick way to change the aspect ratio, you only need to choose a proper effect. Contents is the main thing for every video, so you don’t need to overload it and distract audience. I try to use the effects that direct a viewer's eye to the central part where the main action takes place.

Manual adaptation
Manual adaptation is the best way to change aspect ratio in terms of integrity of a video.

In this case, I create animation layer-by-layer, using precompositions: background, main action, text and marketing materials. I create the most suitable composition for each format. In this case, time is the price for high quality.
Sometimes I combine methods and manually prepare two formats while using a quick way to create the third one (see example below). Here we have a vertical video with a square surrounded with filling.

Almost-automatic adaptation
This way requires you to create a universal layer with the main action. Main elements should remain on the screen in different formats without changes. The layer with marketing materials should be created separately.

In order to better understand which part of the main animation will be cropped in all formats, I create a separate precomposition with the main animation. I put a grid with boundaries of all the three formats on top of it.
If video is dynamic, the audience will not notice the cropped interface and the out-of-frame logotype. The main rule is to keep action visible, closer to the center.
It is hard to predict which video will turn out to be more appealing for the target audience. Often the results of analysis are surprising. There is no universal way to change aspect ratio, as only one of them works out in each individual case. I try to create and cnahge videos as fast as possible to save time. However, in some cases manual adaptation works way better.