How to change framerate with ffmpeg without re-encoding?
This can't normally be done, but luckily for you, MJPEG is a codec for which streamcopy + piping can work

ffmpeg -i in.avi -c copy -f image2pipe - | ffmpeg -f image2pipe -framerate 50 -i - -c copy out.mp4




The JPEG frames in your input are malformed. For such streams, it's a two step process

ffmpeg -i in.avi -c copy frames%d.jpg

ffmpeg -framerate 50 -i frames%d.jpg -c copy out.mp4