One day I'll wish I saved this tip somewhere I could find it... so here it is.
I had a video file which had a stereo audio stream in it, but there was only audio on the left channel of that stream. I wanted to convert that mono-in-left-channel-only stream to pseudo-mono-in-both-channels (ie, copy the left channel into both the left and right channels.
This is the sort of stuff that ffmpeg does before breakfast. What we are doing is "re-mux"ing (which is to say we re multiplex the video and audio streams together into a new container).
ffmpeg -i input-video.mp4 -map 0:0 -c:v copy -af "pan=stereo|c0<c0+c1|c1<c0+c1" -map 0:1 -c:a aac output-video.mkv