Star Wars qualifies due to there being a definite tonal shift between the two trilogies.
^Well, I don't know if a tonal shift is really a genre shift. But yeah, the original trilogy was just a fun homage to Flash Gordon and Saturday matinee adventure serials, while the prequel trilogy was an attempt at a serious historical-political epic with a message about the abuse of power.
How about the Star Wars Holiday Special? That's a genre shift, isn't it?
ETA: And in the related topic of genre-switching TV spin-offs: M*A*S*H and Trapper John, M.D.