I need to modify a movie's displayed subtitles and display as an image overlay
Ok great. So 3 steps really:
1. Make sure you have the .ttf on ur computer installed, if not extract it from the streams

ffmpeg -dump_attachment:t:0 OpenSans-Semibold.ttf -i input.mkv


In my case I had 4 streams and the .ttf was my only t stream so its 0. And the filename to be used is the one listed in the stream info which is OpenSans-Semibold.ttf in my case.


Take the sub file, .ass in this case out of the video container. and save it as a .ass file
Use this command here on the subfile. Open the subfile in a text editor and see the timestamps, and just change the text for a specific timestamp with the formatting you want. And write the time of that timestamps in the -ss. Then after you render out ur output. Just change the .ass in the text editor to have the new text you want. Save. And repeat the command changing the output to something new each time.

ffmpeg -ss 00:01:87 -f lavfi -i "color=color=white@0.0:size=1920x1080,format=rgba,subtitles=subfile.ass:alpha=1" -frames:v 1 out.png