The Five: recastings that actually worked

As per your Superman comments, yes, Noel Neill is remembered as the only Lois Lane, BUT I can never forget Phyllis Coates in the first season because even though I was only a 7 year old, SHE was the Lois who left an impression on me. Coates played a more independent Lois with a more direct reaction to situations, and if you listen to some of the dialogue that she speaks, you will find underneath a kind of sarcastic humour that Neill lacks. I actually missed Coates after she left the Superman show, and still miss her fiesty personality. Too bad that she only played in B movies and bit parts (Leave It To Beaver, Season 1, and Gunsmoke to name a few)after Superman. We sure missed an opportunity of not seeing her more in major parts both on TV and the movies.

I'd have to put in a vote for Carol Burnett and Carol O'Connor as Jamie's parents on Mad About You.
I was trying to remember who perceeded them on the show, but I am no good with names. this is what TV.com has to say on the matter:
After Paul Dooley & Nancy Dussault, and John Karlen & Penny Fuller, Carol Burnett and Carroll O'Connor are the third couple to play Gus and Theresa on the show.

How's that for recasting!?

Since you cheated and used a recast from a pilot episode I'll do the same thing and say that the choice to recast Annette O'Toole as Martha Kent on 'Smallville' over Cynthia Ettinger was a very smart one.

What about Marta 1,2,3 and Ann Veal from Arrested Development?

The original Mama Carlson on "WKRP in Cincinatti" was Sylvia Sydney who was a lot more than the case worker in "Beetlejuice" (among other things she starred in Hitchcock's "Sabotage" and in "Dead End" with Humphrey Bogart. I know from various sources who met her that she could be extremely difficult to work with and wasn't willing to do the role on a regular basis. Inher day she was truly legendary beauty.

Of course no listing of recastings that work would be complete without mentioning "Doctor Who". The part has been recast 9 or 10 times and most of the time it's worked.

In Frank Costanza's first appearance in the episode "The Handicap Spot" of season 4, the character was played by John Randolph. At the urging of series creator Larry David, once Stiller had been cast for Frank Costanza, the scenes with Randolph were reshot with Stiller for syndication. Also, Jason Alexander once revealed that a third (unnamed) actor had briefly been cast in the role circa 1993.

Morty Seinfeld was originally played by Phil Bruns, who appeared in only the second episode. After that episode, Larry David decided that the character as played by Bruns was too laid back, and felt Jerry should have a more crotchety dad. By the time Morty made his second appearance (in "The Pony Remark"), the role had been recast with Martin. When the show went into syndication, Larry David wanted to reshoot Bruns's scenes with Martin (as he had done with the two actors playing Frank Costanza), but decided against the idea because the Bruns episode was too far in the past, and the differences in the other actors' ages would be noticeable.

At least that's what Wikipedia says.

Re Barney Martin (Seinfeld)

I believe they re-shot the seen much later ( a year or two)on with Jerry Stiller, and that is what shows up on TV reruns.

This list highlites just how retooling never works mid season. With one exception, all the references are either pilot episodes, minor characters, or dogs.

Another great pilot retool that worked was Carlson's mother on WKRP. Originally played by the case worker from Beetlejuice, she was replaced by someone who was classier, and without the super gravely voice. Much better choice.

I agree with the nomination of Sarah Chalke in Roseanne. The recasting worked because the scripts really made it funny. They frequently made beyond-the-Fourth-Wall jokes about it. For example, in the opening credits for the last series, they used a morphing technique to age the actors as it panned across the kitchen table. Roseanne's transformations highlighted her bizarre makeovers, while Becky morphed from Alicia Goranson (the original actress in the role) to Chalke and back again (as Goranson had returned).

As for Chalke herself, I thought she was good, but too likable. The 'real' Becky was often snide, shallow and selfish -- as many teenagers really are.

For anyone not sure what I meant by the Fourth Wall:
http://en.wikipedia.org/wiki/Fourth_wall

The mom on "Fresh Prince of Belair" was replaced a few seasons in. In the pilot episode of "The Cosby Show" they only had four children, not five--Sandra didn't exist (not sure if that counts as a replacement though, maybe an addition).