The following command will work:
```ffmpeg -i output.mp4 -c copy output.webm```
Ffmpeg automatically chooses the correct output container format given the output filename by default, unless explicitly overwritten with the `-f [format]` flag.
In this case, since the output file ends in .webm, the WEBM format will be selected.

`-i output.mp4` tells ffmpeg to accept the `output.mp4` file as input, and `-c copy` tells ffmpeg to avoid re-encoding any video or audio.
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