Show of Theseus
A series that's gone on long enough (or merely been so tumultuous) that every element has been replaced or removed, to the point that the first episode and the latest have no cast and little plot in common. Is it still the same show? Do two episodes really belong in the same box set if, with five seasons and only gradual changes in between, one is a light-hearted Dom Com and the other is about post-apocalyptic vampire hunters?
Named for the actually serious philosophical paradox of the Ship of Theseus, which asks whether a vessel replaced plank by plank is still the same boat. Also called Trigger's Broom after Only Fools and Horses. There are even rare examples of philosopher-pranking thieves building new shows out of discarded actors.
Consider this your warning: Since an element of this trope involves cast turnover, it's very possible there will be some spoilers about characters getting written out and killed off.
- Mystery Science Theater 3000 lasted long enough to see every actor walk away for personal reasons. It began as a Mad Scientist and his assistant tormenting a janitor and his robots, but eventually ended as a megalomaniacal woman, a Doctor Zaius Expy, and brain guy tormenting a delivery man and... well, the same robots, but with different voices. Joel, the creator and main host character, left the show to Mike in the middle of season five, neatly dividing the series (and fans) into two eras. Both hosts went on to start their own movie-mocking franchises in Cinematic Titanic and Riff Trax. Both of those shows feature a mutually exclusive subset of Mystery Science Theater 3000's cast (except for Mary Jo Pehl, who has appeared in both series).
- Doctor Who invented a way out of their main character's problematic advanced age - regeneration. Thus the (admittedly alien) William Hartnell became Patrick Troughton and the show was saved. This continued through seven actors, each of them taking along some number of temporary human companions, until unpopular writing and BBC politics killed the show in 1989. The 1996 Made for TV Movie introduced an eighth, then the 2005 Russell T. Davies revival introduced a ninth and tenth in quick succession. Currently the show is on Doctor number eleven, whose introduction coincided with franchise savior RTD's departure.
- Similarly, its original intent was to be an edutainment series. Two of the first three companions were teachers of history and science, and was supposed to actually have much more learning involved. Then the Daleks happened, and the show snapped into a more straightforward sci-fi adventure series.
- Stargate SG-1, almost. Despite three members of the original Four-Man Band making it to the series cancellation (one of them having spent a year dead for tax purposes), most of the surrounding cast and all of the big bads were gone two seasons earlier. Only Walter remained unchanged.
- ER transplanted most of its original cast over the fifteen seasons; while some (Susan Lewis) came back and then went away again, and some of the characters came back in the final season, the final cast was composed of none of the members of the original cast. This even gets lampshaded in the final season. One of the new characters meets one of the original characters and they realize that the staff of the hospital changed so much over the years that they have no common acquaintances. All the people who worked in the hospital when the original character left already stopped working there by the time the new character arrived.
- The talk show originally known as Live! with Regis and Kathie Lee currently has neither. First, Kathie Lee Gifford left the show, and it went on as Live! with Regis until Kelly Ripa was announced as her successor. It then went on as Live! with Regis and Kelly until 2011, when Regis Philbin retired. It's currently called Live! with Kelly until Philbin's successor is announced.
- Guiding Light. It was bound to happen after 72 years, probably more than once.
- New Zealand-produced soap opera Shortland Street first aired in 1992, and as of 2011 only one of the original cast members remains (Dr Chris Warner).
- NBC's Late Night block has gone through this a bunch of times:
- The Tonight Show has gone through at least five different hosts, all of whom brought in their own people to run the show in a new direction. The only exception may be when Jay Leno was brought back to the show in 2011.
- Late Night has gone through a similar situation but with only three hosts - four if you count its predecessor, Tomorrow with Tom Snyder.
- Later: First it was a one on one private interview on a simple set between Bob Costas and another person. After Costas left it became Later with Greg Kinnear with a live audience. A series of rotating guest hosts each brought their own sensibilities to the show. Now it's Later with Carson Daly which is completely different than any of the others.
- BBC Three's Being Human (UK) has had this happen, despite not being exceptionally long and not at all tumultuous. At the end of the first season, the show took a serious Cerebus nosedive. During series 3, the show changed location from Bristol to Cardiff. And finally, at the end of series 4, the last member of the original trio had left, meaning that the entire cast has turned over. This means that when the show returns for series 5 the only thing that remains from the beginning is the original premise of a werewolf/ghost/vampire trio.