The aspect ratio in the browser is incorrect for series

General Discussions Plex Features
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I once made a little bat file to change the language tags on audio and subtitle tracks in mkv files. How to remove tag spam and set language in several MKV files at once

It should be possible to adapt this approach to make it change the properties of the video stream instead.

Totally untested:
Only test it on copies of your video files. It has only a chance of producing valid results on files, which are of identical makeup as your example with s01e01 above. So make sure that you only process the old episodes with it.

for %%f in (*.mkv) do (
            echo %%~nf
    "C:\Program Files\MKVToolNix\mkvpropedit" --edit track:v1 --set --display-dimensions 1:768x576 "%%~nf.mkv"
    )

For all instructions on what to do with this, follow the link above.