Part of this may be the Larry David influence as well. David says his own personality and experiences were a big part of George’s character, and if you look at early George, you can see that too. Of course, you can see a lot of Woody Allen in larry David, too, IMO… 
I think Bart could also apply. He was originally the star of the show (or so it seemed) and was more prone to mischief than he is now (the infamous T-shirt slogan “Overachiever and Proud of It!” is a good example). Now, Homer is the star of the show, and Bart is more of a second banana- still prone to mischief, but not the “bad boy of the '90s” he originally was.
That should be “Underachiever and Proud of It,” of course. “Overachiver and Proud of It” would probably describe Lisa, though 
We’re seeing a change in progress on Desperate Housewives, with Bree’s son Andrew. In the first 2 seasons he was almost pathological in his hatred of his mother, going to ridiculous lengths to sabotage her life.
Now he’s back in a kindler, gentler version; just providing the occasional wry commentary.
Blackadder and Baldrick. In the first series the former was a moron and the latter intelligent. This swapped in the later series.
That doesn’t seem as much like a change in the direction of the writing as it does the natural progression of things. Darlene wore him down to nothing. The same thing happened to Dan only it wasn’t quite as severe and he was always that way on the show.
As for Archie Bunker, he did mellow out, but I think at least some of that was attributed to him being a grandpa.
Whitley Gilbert on “A Different World” made the biggest transformation. During the first season, she was totally unlikeable. Snobbish, cruel, cold, and bossy. She dressed like an old lady and didn’t have any friends. But when Denise Huxtable left the show, she suddenly transformed into a sassy, stylish, redeeming character. She was still snarky, but she was definitely more likeable.
If you compare the first season Whitley with the last season Whitley, you would see two totally different characters.
Eric Matthews, on “Boy Meets World”, went from being an older brother who you could go to for advice if you dared risk being picked on, and a good guy, though not the smartest or the best, to a total idiot savant.
To be honest, I prefer the latter. Eric screaming “FEENY!” manages to make my day everytime I catch an episode.
Oz’s Tobias Beecher wins this one, hands down. He started off as an uptight, heterosexual, alcoholic lawyer who was sentenced to prison for killing a girl while driving drunk. Over six seasons, he became an Aryan’s bitch, took PCP, broke some limbs, went raving mad, screwed (and fell in love with) a few men, converted to Islam… and more!
If it’s 70’s sitcoms I think Margaret “Hotlips” Houlihan certainly underwent a drastic change
from a by-the-book hypocritical harpy to a somewhat mellow (but still by the book) human
female. IIRC there was an ep in like the 3rd or 4th season which found her on the same side
as Hawkeye for a change, against a common enemy, and after that she started to evolve into
her kinder-and-gentler self.
Lucy on St. Elsewhere started out as the meanest, nastiest person on the face of the Earth, fell in love with Ehrlich and became, if not actually nice, at least a good deal mellower.
Norman Buntz on Hill Street Blues was only about a half-step less sleazy than the criminals on the show. He evolved into a rough-around-the-edges but good cop.
Rebecca Howe in Cheers started out as a Type-A personality careerist and devolved into what Diane Chambers would eventually call “the emotional equivalent of Mr. Toad’s Wild Ride.”
On WKRP in Cinncinati Venus Fly Trap eventually morphed from a jive-talking Superfly knock off into a philosophical wise dude who was only slightly farther out than Andy Travis.
As did Radar O’Reilly. Compare season one Radar, who was caught drinking the colonel’s liquor while smoking a cigar and with his feet propped up on Blake’s desk, to the boy who only drank grape soda.
In South Park, Mister Garrison went through a definite change . . .
I want to nominate Radar O’Reilly for worst character change ever. His character started, in the MASH movie, as a brilliant, clairvoyant smartass who ran rings around everyone, cleaned up at poker, lipped people off, and in general was interesting as hell. They could have made a show about that character alone.
But over the years, Gary Burghoff turned him into a stupid bumpkin who could barely read, was shy around girls, and who drank grape Nehis, and who just wanted to go home and live on the farm with his mom and uncle. Bah.
At least Hawkeye remained complex. Radar turned into a caricature.
The change is pretty apparent, but do you have any reason to believe that Gary Burghoff had much influence over this? I’d think that this kind of creative control would’ve been mostly within the purview of Gene Reynolds and Larry Gelbart. Burghoff is said to have publicly expressed some dissatisfaction with his character’s evolution (or lack of):
http://www.faqs.org/faqs/tv/mash/faq/
“In an interview a few years ago, Gary said that he left the show early
because he wanted to pursue other avenues (such as Broadway theatre), and
that he hadn’t been able to develop the character of Radar O’Reilly as
much as he would have liked. However, it doesn’t look like there was any
tension or animosity between him and the staff because Radar was given a
big send-off in an emotion-filled two part episode in the 8th season.”
On Red Dwarf, Rimmer changed from a total smeghead git to slightly less of a smeghead git.
Doctor Who has gone through a change or two over the years.
Alan Shore and Denny Crane both changed between The Practice and Boston Legal even thought the shows are set only a couple of months apart. On The Practice, Alan is self-loathing and deeply unethical while on Boston Legal he’s a wisecracker with a heart of gold. Alan has never done anything on BL that comes close to his antics on TP. Denny was a brilliant but eccentric lawyer on TP but a buffonish caricature on BL. His sex drive also increased about a thousand percent. I actually prefer the Practice versions more as they’re more complex and realistic.
Actually, he was only a jive talker in the pilot episode. That changed immediately.
In a later episode, it was revealed that this was merely a persona that he adopted as Andy brought him on board.
Incidentally, Jennifer initially had an adversarial relationship with the Big Guy, referring to him as “that jerk.” This was dropped immediately after the pilot as well.
I had read somewhere that the O’Reilly character was changed into who he became by Gary Burghoff, that Burghoff was taking the character where he wanted it to go. Sorry, no cite. I was just something I read a few years ago.
Lou Grant changed from an irascible, probably alcoholic, just-getting-by local TV newsroom manager, to a confident, competent, and crusading newspaperr editor.
Trapper John McIntyre had a similar transformation (although this may have been related to personal growing up he did in the twenty or so years between his time in the 4077th to his chief of surgery gig at a major metropolitan hospital).