The main issues are:
- manually splitted versions of the same movie get assembled again
- there can be major mismatches if the file names and folder organization are not according to the recommended Plex standards (mainly with “custom” media types, or if you previouisly used 3rd-party scanners and metadata agents [Anime, Pr0n, etc.])
- automatic collection creation can collide with existing collections (but again, only if the ‘collections’ field hasn’t been locked)
I recommend to not use the automatic upgrade which is prompted by this popup.
Instead, use these articles as reference to do the migration manually.
Then, (before triggering the big Metadata Refresh of the whole library) enter the “Advanced” tab of your library properties a second time.
It should now show all the customizable agent preferences.
Here you should verify and/or customize the settings so that they fit your personal preferences better.
Most important is the “Prefer local metadata” checkbox. The average user wants this OFF – unless he has checked the embedded meta tags of all mp4/m4v files in the library for correctness.
In any case: don’t interrupt the upgrade process.
And only pass judgement on the changes after the final metadata refresh has been finished.
