An inelegant title, I know, but I couldn’t think of how to phrase it. In other words, in early Simpsons episodes, Ralph Wiggum wasn’t the total retard he is now- he was a regular classmate who said normal things, and at some point turned into…Ralph Wiggum. Another example is Max Power, but that’s all I can come up with.
In the first season of Married with Children, Peg Bundy did cook and clean (not that she was any good at it). As the seasons went on, it got to where she never cooked nor cleaned nor did anything that could be vaguely interpreted as ‘work’.
The Fonz got more and more sanitized as the show went on.
Screech from Saved By The Bell
Jesse Chocran (from Full House) changed from a bug exterminator to Jesse Catsopolis the suave greek.
It’s a stretch but Stever Urkel had his alter ego Stephan Ur-Kel.
Darrin Stephens in Bewitched changed quite drastically between seasons 5 and 6.
At some point, drunken, woman-chasing Hawkeye Pierce turned into Alan Alda.
StG
As did Becky from Roseanne - and then she changed back!
My favorite character change is one that was totally intentional in a good way: Wesley, from Buffy/Angel. It’s amazing to me the skill of the writers and the actor to believably take a foppish twit all the way to badass and beyond in a few short years. If you saw his first episode of *Buffy *and then his season four of Angel, you’d never buy it. But in continuity, the change was so well done that it was totally believable.
I came in to talk about Wesley Wyndham-Pryce, my favorite TV character of all time. His character arc starts in season 3 of Buffy and then takes him through all five seasons of Angel, and it is amazing to follow. He starts as this pitiable nerdy stuffed-shirt, too inconsequential to even be considered an antagonist, and ends up as the hardest man in Los Angeles, a tragic hero who changes and grows due to constant suffering.
As often occurs in an actual marriage.
OW!
OW!
STOP HITTING ME!
Sticking with The Simpsons, how about Homer himself? Certainly, his change has been more subtle than Ralph’s, but Homer started as mostly “mean and cruel” (e.g. choking Bart in almost every episode), and has changed to the point where he’s mostly lovably “incompetent and gluttonous.” 
Babylon Five had a lot of (deliberately planned) character change.
For instance, G’Kar started out as a military-minded, arrogant egotist and ended up as a holy man.
Lando Molari was first a buffoon, then a dangerous proto-meglomaniac, and finally, a man trying to do the right thing to correct his mistakes.
Most of the Friends cast changed a little but the worst change was that of Monica. She went from being someone who was just a little bit of a clean freak to being someone who should have been medicated and institutionalized. Her entire character became an over the top cartoon when most of the others became more human, except Joey who lost even more IQ points, but at least he wasn’t a screeching harpy.
Also fron B5, D’lenn made a major physical change, to become a human/minbari hybrid. Lyta Alexander went from fairly ordinary TP to Vorlon Doomsday Device with Evil Red Eyes.
At the risk of inviting the wrath of Buffyites everywhere, one change I was NOT
pleased about was Willow (who was once my favorite character on BTVS). I say
this not as a homophobe (and actually Tara was pretty cool), it’s more along the
lines of an inexplicable character arc which annihilated my suspension of disbelief.
I seriously doubt that Whedon originally intended her to “go gay” from episode 1,
and it indeed has all the hallmarks of something foisted onto the character halfway
through the show without any regard for consistency (or even aesthetics for that
matter).
In addition to Hawkeye Pierce, MASH’sHot Lips Hoolihan, Radar O’Reilly, and Max Klinger all changed their personalities over the years.
Eustace Scrubb turned into a dragon, and then a likeable human being.
“Max Power” was a nickname Homer gave himself in a Simpsons episode (ironically, after a TV character named Homer Simpson was changed from a cool guy into a Homer Simpsonesque buffoon). Was there actually a TV character named “Max Power” on some other TV show?
I think it’s the TV character of Max Power (the cop_ who Wee Bairn is referring to - suave bullet-catching cop to accident-prone oaf.
While Willow’s drastic change didn’t ruin by suspension of disbelief, I didn’t like it either. I didn’t like the way they pivoted her character based on what appeared to me to be a throwaway line, “and I think I’m kinda gay.”
Absolutely agree. And I also loved Tara (named my cat after her, in fact), and loved their relationship. A bisexual Willow, I’d have had no problem with. I wouldn’t even have cared if she “discovered” she was bi in college, but it would have been rather cliche. I don’t like that she “caught” Teh Gay after her relationships with Xander and Oz. It diminished what was a fantastically beautiful relationship (the Willow/Oz one, I mean) by having it be less than her ideal. And I really hated the episode where (when everyone was under the influence of the letterman’s jacket and wanted Xander) she wanted to turn Xander, her old schoolgirl unrequited love, into a girl because she’s “gay now!” What the fuck ever, biotch. You liked dick once, that just doesn’t go away when you discover pussy.