@mshe You could make use of PlexCSV to detect which entry of your library was identified using what agent (MetaDb field).
I did that on my libraries (changed from MovieDB to PlexMovie) and found all old MovieDB items, checked on IMDB for their IMDB ID and did a new identification by IMDB ID of those items only.
If this makes any sense to you depends on the number of “old” items you find 
Of course, doing a Plex Dance with your whole library is another option you have.
