ETA: And in the related topic of genre-switching TV spin-offs: M*A*S*H and Trapper John, M.D.
Technically, Trapper John, MD
is considered a spinoff of the movie M*A*S*H
rather than the TV sitcom -- although that was just done to avoid paying royalties to the sitcom's producers. But yeah, I guess both shows derived from the movie ended up going in a more dramatic direction, though in the case of M*A*S*H
the series, it was a more gradual transition.