There are soap operas that started out on radio; surely some of them had complete turnaround in the cast!
Well, heck, Saturday Night Live has been in steady turnover since its early years.
Stefan Dennis is still there, apparently (or has been back for some time after leaving for a bit – does that count?)
Taggart would seem to fit the bill; none of the current cast were in the earlier episodes.
SNL started in 75 and in 80 they had an all new cast. Those early 80s shows were pretty bad until Eddie Murphy and Joe Piscopo came along.
Wow, I get to be the first to mention MST3K.
Apart from:
Never mind – maybe next time. 
I can honestly say I’ve not seen Susan in years. She was sitting on the steps with the rest of the cast at the end of the 30 Year special. That was the last I’ve seen her.
You know, I read every post in this thread, and yet there it is…
Huh.
I blame the fact that it’s still before noon.
Charlie’s Angels was down to only one original Angel at the end so thats a 66% cast make over.
The British show Spooks, aka MI-5, were losing cast members at a rate of at least one major character per episode by about series three or four. I stopped watching around then, but surely they can’t have hung on to anyone original by now.
Oh, and Monarch of the Glen also qualifies, I believe.
Skins changed casts after 2 years. The idea was to follow students in their last 2 years of school. When they finished school the show started to follow a completely different group. No 28 year old teenagers staying in school year after year.
It didnt completely change cast though. One of the main characters in the 2nd batch is the sister of one of the main characters from the 1st batch. Although she only appeared minimally in the 1st two series.
The plan seems to be to follow this by replacing everyone every 2 years. Provided the show sticks around that long.
I’ve watched a fair number of British TV series on BBC America over the last several years and I’ve noticed something: British TV series have no compunction about killing off main characters (or otherwise removing them from the show).
In US TV shows, you know the main characters are going to get out of the jam they’re in because we almost never kill off a character. On British shows, that’s not the case.
Here are some British series that come to mind that have done this:
Torchwood
Doctor Who (of course)
Primeval
MI5
Not Going Out
I’m not sure why this is. Do British actors want more variety? Does it have to do with the British film industry? (Newer actors on a series are cheaper than long-term actors?)
Does anyone know?
J.
American shows will kill off a character if the actor wants out. Some examples :
Jean Stapleton from All in the Family
Jimmy Smits from NYPD Blue
McLean Stevenson from MASH
Maybe British actors do not like to stay on shows for very long.
My WAG is that it’s an effect of the British system, in which, rather than being ongoing and open-ended like American shows, British TV shows are presented in the form of “series” of a relative handful of episodes. Once the “series” is over, there may or may not be another, and there may be a substantial gap of years between their production.
The Tonight Show with Steve Allen, Jack Paar, Johnny Carson, Jay Leno and Conan O’Brien.
Late Night with David Letterman, Conan O’Brien and Jimmy Fallon.
And here’s one that’s suitable for a bar bet. In 1949 NBC aired The Life of Riley starring Jackie Gleason. The show lasted one season, but came back in 1953 with William Bendix as Riley and a completely new cast. That version ran for 5 years, so does it count as one show with a completely different cast or two different shows?
They didn’t kill her off until sometime after it morphed into Archie Bunker’s Place.
MAS*H came close, with only Alan Alda and Loretta Swit left by the end.