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This works to change the speed, but it also changes the codec to MPEG4 Video (H264).

ffmpeg -y -r 16 -i at25fps.mov -filter:v "setpts=25/16*PTS" at16fps.mov

I need the codec to stay untouched: prores (HQ) (apch / 0x68637061), yuv422p10le(tv, progressive).

I found many similar post, but nothing worked: How to change framerate with ffmpeg without re-encoding?, Lower framerate without re-encode

This post suggest I first extract the raw stream - how would I do that for a ProRes file?

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I just found a way to do this by changing the parameters help by fps tbr tbn tbc; answer is derived from this post.

ffmpeg -y -i at25fps.mov -c:v copy -video_track_timescale 16 at16fps.mov

I am getting these remarks from Apple Sound Media Handler (so I am not sure if this is the best way to do it)

Non-monotonous DTS in output stream 0:0; previous: 0, current: 0; changing to 1. This may result in incorrect timestamps in the output file.

Non-monotonous DTS in output stream 0:0; previous: 1, current: 1; changing to 2. This may result in incorrect timestamps in the output file.

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