Change frame rate without changing film speed/duration

I've made a mistake and I need to re-encode video from 30 fps to 24fps, while maintaining the same speed/duration.

Does anyone know how I might do this in FFMPEG?

I've tried changing the -r value and this changes the rate, but changes the video length.

Thanks

To change the frame rate without modifying the total length of the video, FFmpeg has to duplicate or drop some frames. Unfortunately this process can turn a steady smooth movemen to become clumsy and unnatural in the video.

ffmpeg -i input.mov -r 24 -y output.mov

This method is very slow so be patient. The audio will remain the same, since length does not change.

You can also check this tool if your results are not what you expected: MVTools

Here is a thread that may be helpful Change framerate in ffmpeg without re-encoding

This command won't change the video playback speed:

ffmpeg -i <input> -filter:v fps=fps=30 <output>

Worked well for reducing fps from 59.6 to 30.