Upgrade, if your media are “match-able” against Plex’s metadata sources
(movies: IMDb and TheMovieDB,
tv shows: TheTVDB and TheMovieDB)
But do it “manually”, do not use the popup.
Refer to the articles linked below.
After the conversion, but before performing the metadata refresh,
adjust the agent preferences (on the “Advanced” tab in the library properties) and disable automatic collections.
https://support.plex.tv/articles/upgrading-a-movie-library-to-the-use-the-new-plex-movie-agent/
https://support.plex.tv/articles/migrating-a-tv-library-to-use-the-new-plex-tv-series-agent-scanner/
