This procedure might help you → Reset all manual poster selections in a library at once or unlock other metadata fields
It will reset the locked fields for thumbs and can be done at various levels including movie, show, season and episode depending on your needs.
Might be worth a try as it should reset the fields for the thumbs and then doing a force metadata refresh should get plex to download new ones. It would however blow away any your manually selected as it resets the locked status might worth a look at.
