In addition to the Plex dance, a couple other options would be to merge it into an “older” show, then split it apart and rematch it, or use the API (e.g. via Python-PlexAPI’s editAddedAt method) if you’re comfortable with scripting.
As for why changing the file dates didn’t help, Plex will only use the file date during the first scan of a new library. Everything after that will use the date the item was first scanned in as the Date Added.
