Wesley Wyndam-Price from Buffy and Angel starts as a buffoon and ends as a serious and heroic character.
G’kar, Londo, and (to some extent) Vir (all from Babylon 5) all undergo transformations over the course of the series, with G’kar transforming from a revenge-seeking revolutionary to a saintly religious figure, Londo transforming from a minor diplomat to a genuine statesman, and Vir going from a buffoon, to a statesman as well.
Oddly enough, in “Sliders” the character of Remington (Crying Man) goes from being a joke to being the heart of the group of sliders.
Michael Corleone.
Even though his family was doing criminal things he swore off of it and wanted to live an honest life. Then like Walter he found himself in a bind and talked himself into commiting crimes “just this once” because “it’s for the family”. Next thing you know they’re a mob boss / drug lord.
Not a total character shift over the course of a series…but…
There is an episode of Deep Space Nine where Sisko gets sucked into some pretty shady shit morally speaking…but it is for the greater good…and he apparently can live with it.
Over the course of I forget how many shitty Gor books, Tarl Cabot goes from being a John Carter-style hero to an anti-hero to, arguably, an evil man. This was driven mainly by the author letting his personal kinks drive the series. A similar arc, for the same reason, happened to Richard Rahl over the course of I forget how many shitty Sword of Truth books.
Al Swearengen on Deadwood.
He goes from being set-up as the antagonist/villain to being a monster with a heart of gold to being an agent of order (but not necessarily of the law).
I wholeheartedly disagree with Walter White, actually, and a lot of the other examples.
Part of what’s makes Breaking Bad so cool is that Walter White is who he is for basically the entire show. His ACTIONS change; he goes from being a shy teacher to murdering people. his fundamental character, however, is largely unchanged; what you’re seeing is the fact that he always was a deeply flawed and ugly person inside. In the spirit of a good Shakepearian drama, he is a man of considerable talents but who has a tragic flaw - prideful arrogance - that ultimately is what determines everything he does. Everything he does, that brings him up and tears him down, comes from pride.
Michael Corleone, similarly, was a cold, calculating bastard the entire time.
Most character transformations are the result of deliberately changing things or just a character changing over time - Homer Simpson started out as a fairly conventional character, not the retarded buffoon he became. Or Ash Williams, the central figure of “The Evil Dead,” who started out as a normal person but later became a wisecracking, dimwitted clown. That just served the purpose of the vehicle, it wasn’t really a change of character.
For a good character change I am reminded of the character of Rubin Carter in “Hurricane.”
Captain Burt Ramsay on the soap General Hospital. He was the chief of police, a supporting character who was reasonably good. Then a lead character who was a former spy but had no real police experience was promoted over him to police commissioner. So, Ramsay became the head of organized crime in Port Charles, while still acting as police chief so he could divert attention away from organized crime.
Edmond Dantès -> The Count of Monte Cristo
Pinocchio -> A real boy!
Gerry Conlon (at least in the film In the Name of the Father)
Leonard “Gomer Pyle” Lawrence
So then, what about Gaiman’s Lucifer Morningstar? Once loved above all other angels (IIRC–it’s been a long time since I’ve read The Sandman) to being the Devil? Didn’t pride have a lot to do with his Fall? (Again, I’m just going from years-old memory.)
How about Rick from the Walking Dead? I’m just tossing it out there and I might be totally wrong. In fact, many of the characters went from level headed ‘normal’ citizens to ‘KILL EM ALL’ alive or not.