iTunes to Music migration to Catalina: playlists become corrupted 'smart' folders

Thank you turingtest2 ... using a backed up version of the .itl music library and the option-start-Music method did indeed give 'dumb' playlist folders where intended, so the conversion has gone better as you suggested. The backup dated from last September when I migrated from iTunes to the Music app, and the problem with the corrupted library was not apparent at that time. I only noticed it during the last month or so, but that might just be because I haven't been actively adding to or editing my music collection - just playing it, or it might have happened recently as a result of some more recent software update. There were only 4 albums and playlists I'd added in the intervening time. Their tracks were all present in the Music Media folder so I just had to import them into the library again and all is now as it should be.

The exercise has been useful though because I've now checked that the library knows the correct location of all the tracks. There were a very few that had unaccountably moved from their correct location in the Media folder hierarchy and just needed moving back and 'locating' when prompted by the Music app.

Thanks again.