I’d bet, also, the upbeat theme music of early seasons was TV’s version of the mise en scene, a subtle reminder to TV execs and viewers alike that M*A*S*H ostensibly was a comedy.
In those days, black comedy on the small screen freaked out your average TV exec, and that’s why the tone in the first few seasons was much lighter than the Altman film. Once CBS brass were assured of solid ratings, the producers and writers were freer to go darker, especially as the country got farther away in time from the Vietnam War, which was what Altman’s film was really critiquing.
Huh, whaddya know? My TV/film degree is actually good for something once in a while!