plex (apparently) lost gracenote, and recently became a musicbrainz sponsor.
they haven’t finished the transition, so in the mean time metadata and other features will be spotty or non-existent.
perhaps instead of making more work for yourself by re-doing your library, you should just be patient (along with the rest of us) while they migrate to the new provider(s).
see also @ Important information regarding Premium Music libraries in Plex
