“Hana Ha Odoreya I-Ro-Ha-Ni-Ho” starts with a pentatonic melody in Fm key.
Now listen to the chorus part at 0:52. The same melody is repeated, but the accompaniment is in Cm key. The opposite pattern as yours; the modulation to the dominant key.
The point here is that the melody is pentatonic. The tones are C, E♭, F, G and B♭. This is a well-known C minor pentatonic. So from the start it kinda holds bitonality—C minor pentatonic superimposed on F minor key chords. But it’s not much recognizable because all the C minor pentatonic scale tones are the part of the F minor scale.
The difference between Fm key and Cm key is whether to have flat on D. But the melody does not have it. This enables the bass in the opening to play D♭, realizing “retonalization”.
Pentatonic scales may be able to play an interesting role on implementating this technique.
