Files are imported to the beets database using beet imp foo.mp3. In my case, this triggers the LastGenre plugin to apply genre tags automatically. The imported files are stored in a directory structure based on the aforementioned genre tags (eng/rock/artist/album/foo.mp3)
If the genre tags are inaccurate, they can be corrected using beet modify "foo" genre="bar", but how do I tell beets to also update the underlying directory structure?