Swapping video sides of side-by-side clip with ffmpeg
There may be an easy, quick way out, which is to remux to Matroska and set metadata to indicate that the right view is on the left.

ffmpeg -i in.mp4 -c copy -stereo_mode right_left out.mkv


Upload to Youtube and check.



Else, FFmpeg has a filter to handle this directly.

ffmpeg -i in.mp4 -vf stereo3d=sbsr:sbsl -crf 17 -c:a copy out.mp4


The filter interprets the input as side-by-side with right eye view on left side (sbsr) and swaps the halves.