iTunes Match DRM: Update Finder Music files

Since I subscribed to iTunes Match last year, all of the DRM protection has been stripped from the songs in the iTunes Library on my Mac Mini. However, some of the song files stored in the Music folder in the Finder are still showing a protected status for many songs I bought before DRM went away. How can I update the Finder files so that they contain the unprotected version of the songs in the iTunes Library?