How to change the container/format of the video without re-encoding the video?

You can change the container/format of your video file without encoding if and only if your target container/format supports the video and audio codec of your existing video. Otherwise, you will have to re-encode the video with a new video/audio codec supported by your new container/format.

In your case, WebM format does not support H.264 video codec so you have to re-encode your video to either VP8 or VP9 codec which is supported by WebM format.

Refer to
these links for more information in detail:

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