How do I speed up slower video framerates with ffmpeg and get full 240fps and no truncated videos?
Use

ffmpeg -i input.mov -vf "setpts=N/240/TB" -r 240 -crf 18 output.mov


The only thing the setpts filter does is retime frames, but the designated frame rate of the stream is still its original framerate, whatever that is (25,30..). Use the fps filter to change that - the fps filter may also drop or duplicate frames, but not in this case. There's a bug in the fps filter which makes it ineffectual for this purpose. Use output -r.

The default encoder for MOV in FFmpeg is libx264. Its default ratecontrol mode is CRF. Lower values retain more of the original quality but produce larger files - 18 to 28 is a decent range. This has no effect on which frames are included in your output.