I would also like to see Plex automatically regenerate thumbnails when the media file is modified. I overwrite media with newer, higher quality versions when they are available. However, the thumbnails do not get regenerated when the media file is replaced with a newer version. This causes the thumbnails to be out of sync, especially if the new media is an extended version or director’s cut. It would be nice if Plex Media Server would automatically regenerate thumbnails when it sees that the media file has changed.

I have only found a couple of ways to regenerate the thumbnails.

The easiest way is to delete the media and clean the bundles. However, this will put the movie back to the top of the “recently added”.

Delete the file from the library folder. Rescan the library. Once the movie is removed from the library, select the “clean bundles” option. This will delete the media files along with the preview thumbnails. Add the movie back the library folder. Plex will add the movie back to the library and regenerate the preview thumbnails.

If you don’t want the movie to show up at the top of “recently added”, you can use the Plex Media Scanner command line options:

https://support.plex.tv/hc/en-us/articles/201242707-Plex-Media-Scanner-via-Command-Line

Here are the commands I ran on my Mac Mini:

List the Library Section IDs:

/Applications/Plex\ Media\ Server.app/Contents/MacOS/Plex\ Media\ Scanner -l

1: Movies

2: TV

List the items in movie section (section ID 1)

/Applications/Plex\ Media\ Server.app/Contents/MacOS/Plex\ Media\ Scanner -l -c 1

321305: Star Wars

Regenerate thumbnails/fanart for the movie in question (item ID 321305)

/Applications/Plex\ Media\ Server.app/Contents/MacOS/Plex\ Media\ Scanner -g -o 321305

Generate media index file (item ID 321305)

/Applications/Plex\ Media\ Server.app/Contents/MacOS/Plex\ Media\ Scanner -b -o 321305

I have also created and attached a couple of perl scripts for OS X that will regenerate the thumbnails/fanart for all movies and/or tv shows. These scripts will create a new bundle if the media has changed since it was originally scanned. Once these scripts have been executed, the thumbnails will be regenerated the next time the library is force refreshed or analyzed.