Even if you tell ffmpeg that the output file is 240fps, the ReplayMod will still output only the frames you specified in the framerate settings. You shouldn't mess with the output settings.
While there's really no point in outputting 240fps (YouTube only supports 60fps footage anyway) and you should use the time keyframes instead to make the video slower, you can edit the "replaymod-render.cfg"
in the .minecraft/config
folder and adjust the "framesPerSecond"
value as you wish.
After restarting Minecraft, the value will be set to 240 and a 240fps video will be rendered (for whatever reason you may need this).
I tested it but the replay mod always resets the value to 120fps in the config file.
So every time I hit render my video framerate is reseted.