M*A*S*H went through multiple different phases, rather than just "early" and "later" episodes/seasons. You've got Seasons 1-3, which were the Trapper/Henry/Frank seasons; Seasons 4 and 5, the B.J./Potter/Frank seasons; Seasons 6 and 7, the B.J./Potter/Charles seasons; and then Seasons 8-11 without Radar.
Personally, I found M*A*S*H got better with age. The first three seasons dealt with a lot of network meddling and scrutiny, which is why they were a lot sillier and light-hearted, since the network didn't want the show getting too dark or too gory - it was only after the show proved to be a hit did the network start backing off, giving the production staff more creative freedom, and as such, the humor was less wacky anti-war satire, and more sophisticated and character-based. Not to mention, they eventually found a better balance of comedy and drama to the point where one didn't overpower the other. Unfortunately, but the time we get to Season 8, then everything changed: the production staff was overhauled, new writers were brought in, the drama was heightened, and the comedy took a serious backseat - that's when I would say M*A*S*H jumped the shark.
IMHO, I would consider Seasons 1-3 to be really good, Seasons 4-7 great, and Seasons 8-11 terrible.