I think Cheers might be one of the better examples here, though being an ensemble (much like many of the others mentioned, its a theme) makes it less surprising.
Over the years they replaced Coach with Woody and Diane with Rebecca, both were largely seemless transitions.
Another which is probably the pinnacle of this thread would be Law and Order. the show has been recast so many times it’s absurd, though rarely with any remarkable downgrading. The Diane Wiest casting nonwithstanding.
Lassie went from being about a smart dog who continually rescues its human family to the adventures on a smart dog on its own.
St. Elsewhere transitioned pretty seamlessly from stories about a bunch of older doctors teaching a bunch of younger doctors to a bunch of somewhat experienced doctors pretty much on their own.
Happy Days was much better after Fonzie jumped over a shark on his motorcycle.
If cast changes count, I’ll say Saturday Night Live when they replaced Chevy Chase with Bill Murray.
And that’s the only freaking cast change that applies.
LALALALALALALA! I can’t hear you.
I like the new assistant on Monk way better than the old one. She’s a much more interesting, complex character, unlike the old one who was simply abrasive. Then again, the new assistant was only in the last five episodes or something, so maybe it’s not a permanent good change. But so far I think it is.
Many of mine have already been mentioned: Law & Order, NYPD Blue, Babylon 5, MAS*H (I’ve been debating whether to buy MASH on DVD – I’ve concluded that if I do, I will start with the arrival of Potter and B.J. I’m not interested in the earlier episodes; the later ones made it my favorite show. Of course, I’ve heard people expressing exactly the opposite opinion).
Although it’s probably in the Charles in Charge “was it ever good?” category, I’ll add The Facts of Life. When it focused on the school as a whole, with a large group of girls, it was a horrible, D-grade sitcom . Once it dumped everyone but four key girls and concentrated on the development of their personalities, it became much more entertaining and survived for nine years (although eventually it went through additional changes that were not for the better).
Remington Steele improved tremendously once they ditched the unmemorable first season assistants and brought on Doris Roberts.
JAG was cancelled after its first season, but made some cast adjustments when it was picked up by a different network and went on to run for ten years.
Transformers changed a lot for its third season in 1986. The first two seasons were about warring factions of alien robots on Earth in the present day (1984-85). They made friends with humans, and disguised themselves as realistic-looking vehicles, to blend in on their new planet.
Transformers: The Movie came out in theaters in the summer of 1986, and flashed forward to the year 2005, where the war had stepped up in intensity and violence, the Decepticons (bad guys) were winning, and a whole new group of Transformers that changed into flashy, futuristic vehicles was introduced. The movie was a lot darker and more “mature” than the two previous seasons, and the third season that followed it took place after the movie. The Autobots and Decepticons experienced more casualties, explored their own mythology, and discovered the truth behind their creation. Some of it got pretty heavy and archetypal for an after-school cartoon for kids.
I think Alias is still a good show, despite the changes it has gone through. I loved the addition of Lena Olin to the cast, as Irina, and have enjoyed watching her interactions with Sydney and Jack and, now, Nadia. I love that I still can’t decide whether Sloane is a good guy or a sublimely evil guy. Each time that I think it’s finally safe to unreservedly hate Sloane, he does something that is truly good.
I realize it’s not to everyone’s taste. I’m just saying that I think it is still a good show despite the defeat and dissolution of SD-6.
X-Files changed rapidly from its fifth season onwards. Where the episodes in seasons 1-4 were dark, suspenseful, and horrific sci-fi, you started to see comedy-oriented episodes creeping in around season 5–“Post-Modern Prometheus”, for example. Season 6 had more and more of them (see “Triangle” and “The Unnatural” for an example), and Season 7 was almost all comedy episodes.
This was a good thing, if the writing stayed strong–but it didn’t. And David leaving the show around Season 8 put the final nail in the coffin.
If you liked the movie you should at least get the first season. From the second season on, the scripts were decidedly less movie-inspired.
I’d call this a toss-up, at best. A sizable number of the Transformers old-school fanbase hated the third season, for numerous reasons:
[ul]
[li]Crap-asstic animation[/li][li]Shorter episodes (as another cost-savings measure)[/li][li]The further dumbing down of Grimlock[/li][li]Hot Rod, the Cybertronic whiner[/li][li]Emasculating the Decepticons (especially with Galvatron The Nutball in charge)[/li][li]Overreliance on the ineffectual Quintessons as a “threat”[/li][/ul]
It’s hardly a surprise that the most unpopular Transformers cartoons tend to be from the third season, especially “Five Faces of Darkness,” the five-part season opener.
I have to go with The Daily Show. When I first heard Craig Kilborn was leaving, I thought it would be terrible - Craig Kilborn WAS the show. Now, I know the beauty of Jon Stewart, and I can’t imagine how I ever liked it with Craig.
I’ll cite Yes, Minister / Yes, Prime Minister
Seven of Nine 
Well, of course:dubious:
Any episode was better than getting the shark! 
The Odd Couple. After the first season, they got a whole new apartment set, the silly Pigeon Sisters disappeared, and most of their poker buddies disappeared (Murray the Cop being the big exception). And come to think of it, I think they pretty much stopped playing poker. The show was much improved.
When Life Goes On ceased to be a show about “a family with a retarded kid” and became a show about a family- with Corky no longer the main focus.
Then it became better by leaps and bounds by exploring the story of the Becca’s HIV+ friend, Jessie. At the time, this was over ten years ago now, it was actually pretty daring to deal with AIDS in mainstream entertainment- even moreso in the case of Life Goes On, which was not only mainstream but a Sunday night “family” show.
I have to second Winkie’s nomination of The Daily Show. It went from being just another Comedy Central time filler under Kilborn to being the best damn political humor going in the U.S. under Stewart. The show didn’t just survive, it improved by an order of magnitude.
Talk Soup is more in line with most of the shows in this thread – it survived the loss of Greg Kinnear, then the loss of Rat Boy, etc. I’m not even sure who’s doing it now, but he’s about as funny as the rest of them.
Oh, and America’s Funniest Home Videos improved dramatically when they brought on Trace Beaulieu and Josh Weinstein from MST3K to do the writing. The clips are still the same, but they’re packaged with much more intelligence and drollery. Plus I’m glad to see the MST3K guys get a nice-paying gig, far cry though it is from MST3K.
When it first started out, Ellen was entitled, “These friends of mine.” Then the writters changed the tone, focused more on her, and changed the title, and I loved it!