Audrey Conner (ROSEANNE)- She made an appearance in season 2 and she was a successful travel agent dating a new man. Toward the end of season 6, they talk about how crazy she has gotten and have to have her hospitalized for mental issues. She makes one last appearance in season 9 still mentally unstable.

Stephanie Tanner (FULL HOUSE)- She was witty and fun loving from seasons 1 - 5. Season 6 onward she turned into a moody brat and teenager.

Kimberly Shaw (MELROSE PLACE)- She was "the other woman" for Michael Mancini in the beginning of the series. She leaves for a while and comes back toward the end of season 2 turning into a raging lunatic kidnapping a baby, trying to kill Michael and various others. Then she does another 180 and turns into a caring, sensitive person in season 4 (She even does therapy sessions for Sydney, the woman that Michael cheated on her with!)

Michael Mancini (MELROSE PLACE)- He was the sensitive guy in season 1, the person everyone could go to with their problems. Toward the end of the season, he cheats on his wife and turns into a selfish jerk out for his own gains.

Jo Polniacheck (sp?) (THE FACTS OF LIFE)- When she comes to Eastland in season 2, she was a trouble maker and a slacker. As the series progresses, she cares more about school and is even the class valedictorian. She even becomes a social worker toward the end of the series.

Topanga Lawrence (BOY MEETS WORLD)- She was the hippie nerd girl in season 1 that Cory and Shawn tried to avoid. Season 2 onwards, she was the all American girl.

Steve Urkel (FAMILY MATTERS)- Was a complete nerd then towards the end of the series, then became a suave guy (ended up marrying his long time girl crush Laura Winslow).

I agree about everyone except Stephanie Tanner. I dont think she was a moody brat. I think she was just growing up.

What about Buddy Lembeck on Charles in Charge? He was pretty straight forward when the series first started and became annoying and over the top as the series went on.

Eric Matthews from Boy Meets World. See Buddy Lembeck.

Samantha Kaniski (Gimme a Break)-She was a tomboy for the first three seasons, but by seasons 4 and 5, she outgrew the tomboy scenario...her older sisters Katie and Julie...stayed the same...though Julie married (eloped) someone who was like her sister Katie.

Margret "Hot Lips" Houlihan (MASH)--She started out as an unlikable b.... (you know what I mean!) who had an affair with a married man, but by the end of the show became very likable.

Nancy Oleson, Sue Ann Weaver, and Cindy Webster from "The Facts of Life". Once they were three nice girls who frequently defended the other girls in their dorm; they later became gossipy and kind of mean in Seasons 2 and 3, implying Natalie was a slut, spreading gossip about Mrs. Garrett, and (in Nancy's case) competing against Blair in a modeling contest. Then they stopped visiting the cafeteria altogether.

Not sure it was a '180' But Chrissy on "Threes Company" got more scatterbrained as the seasons progressed.

Not sure it was a '180' But Chrissy on "Threes Company" got more scatterbrained as the seasons progressed.
TV Tropes calls this Flanderization (http://tvtropes.org/pmwiki/pmwiki.php/Main/Flanderization). Tragically, so many sitcom characters come down with this incurable affliction partway through a series' run.

Not sure it was a '180' But Chrissy on "Threes Company" got more scatterbrained as the seasons progressed.
In the first 2 seasons, she wasn't that dumb. She had some brains.

Jackie Harris - Rosanne: started off as a smart, thoughtful, funny woman who much later became a total goofball who made no sense toward the end of the series.

J.R. Ewing-DALLAS

for most of the series, he was a really evil, nasty, son-of-a-***** But then in the last couple of seasons, he became less evil and nasty. (and the show suffered as a result)

Zackary Smith, Lost in space. The character was so serious the first few episodes and then was a easy going getting of work type.

Suzanne Sugarbaker - Designing Women:

She was a lot more intelligent and well-spoken in the beginning of the series. By the time she left, she was loud, crass, tactless and stupid, but the change in her character had to do with the behind-the-scenes feud with the show's creators.

Rebecca Howe - Cheers: In at least her first season (Season 6), she was more cold and semi-hostile to Sam. Afterwards she became more open to him.

As for Buddy Lembeck on CiC - he started off a somewhat wiser and smarter on the CBS season and 1st syndie season. From the 2nd syndie season on, he got progressively dumber.

Homer Simpson: Dumb but well meaning and good natured to being self centered, self rightgous, or better know as jerkass Homer.

Randy Taylor: Funny, sarcastic, smart alec joker to a socially conscious vegetarian environmental snob.

dan connor roseanne in the first few seasons he was a kind and loving husband and father but as the series went on he became very rude and selfish and by the end was almost totally unlikeable

Homer Simpson: Dumb but well meaning and good natured to being self centered, self rightgous, or better know as jerkass Homer.

I have to agree with Homer Simpson.

In the early seasons (or the Klasky-Csupo years), he wasn't nearly as dumb as he is now.

Theo Huxtable - Was very lazy and doing poorly in high school. By the end of "The Cosby Show" of course, he was a mentor for inner city students and graduating college with honors.

Marcy Rhoades/D'arcy - Was extremely timid and shy during the first seasons. Her first husband, Steve, was the man of the house. By the last few seasons of "Married With Children", she was a feminist blowhard who didn't bite her tongue about her hatred for the Bundys and kept her foot on Jefferson's neck.

Kelly Bundy - During the first couple of seasons, she was just a typical teenager who was merely naive. By the end of "Married With Children", I would be surprised if she had more than 3 brain cells to work with.

Daphne Moon Crane-Frasier
At the beginning Daphne was the physical therapist to Frasier and Niles father, Martin. She was funny, sweet and a bit eccentric. Near the end of the series she married Niles and became stronger. She was tougher and stood up for herself more. A lot of people didn't like her new personality but I embraced it. It was nice to see her as a stronger person and not afraid to stand up for herself. She was still sweet and funny but not a pushover anymore. Niles love changed her for the better. It was almost like they were meant to be together.

Niles Crane-Frasier
At the beginning of the show Niles was neurotic and stuck up. After he divorced Maris and married Daphne, he was less stuck up and neurotic. He was less wimpy as well. He stood up to Mel, his second wife, which is something he was never any good at. He had always loved Daphne from afar and was finally brave enough to tell her how he felt. Daphne's love changed him for the better. I think he always knew Daphne was the one for him, it was just a matter of time.

Andy Taylor-The Andy Griffith Show
Andy was a nice, funny, smart guy who ran the town of Mayberry without carrying a gun and being too strict. After Don Knotts left the role of Barney, Andy became more grumpy and less funny. He wasn't happy and neither were the viewers.

Arthur "The Fonz" Fonzarelli - Seasons 1 and 2 - A juvenile delinquent and tough talking hood, much like James Dean or Marlon Brando. Season 3 onwards: Still tough enough to not be messed with but gradually showing more of a softer side, until near the end, he turned into more of a normal human being, becoming a teacher and single father and a well-respected citizen. Die-hard fans of "Happy Days" became upset by this and started tuning out the series about halfway through his transformation.

Naomi Harper - Mama's Family:

On the network run she was smarter, had a job as a cashier and more hardened. When the show went into syndication, she became a sex-crazed bimbo, was lazy, and her voice became high and whiny. I watched the syndication episodes first so it was a shock to see Naomi speaking softer with less of an exaggerated accent and being more tough-talking and holding her own against Thelma.

What about Stewie Griffin from "Family Guy?"

He went from a evil genius hell bent on killing Lois to a supposedly hip metrosexual that wouldn't harm a fly.