The question is quite simple. Which TV series continued on despite many of its regulars leaving and/or being recast. In effect, the cast from the first season was radically different from the one from its final season.

For example, among current shows, Grey's Anatomy has like three people from the original cast (https://ew.com/tv/greys-anatomy-cast-exits/) left (https://screenrant.com/greys-anatomy-season-1-original-characters-in-season-17/) and only one of the original interns. Meanwhile, on NCIS, Gibbs and Ducky are the only major roles (https://www.nickiswift.com/298529/why-so-many-stars-left-ncis/) left from the first (https://rare.us/entertainment-and-culture/ncis-cast/) episode.

The Facts of Life started with Charlotte Rae, John Lawlor, Jenny O'Hara, Molly Ringwald, Lisa Whelchel, Julie Piekarski, Julie Anne Haddock, Kim Fields, Felice Schachter, and Mindy Cohn, but ended with only Whelchel, Fields, and Cohn left from the original cast (and Mindy Cohn phoned in her final appearance).

Doctor Who's entire original cast was replaced by the early part of Season 4.

The original Law and Order literally replaced its entire cast that had started out at the beginning. I don't watch soap operas, but with the several decades they have been on the four soaps that are left has to be all replacement cast members from the start.

There are many shows that fall into this category (surprisingly), here are a few:

Lassie
Mission Impossible
Wagon Train
Petticoat Junction
Gimme a Break
All In The Family
My Three Sons

The Andy Griffith Show
Bewitched
Happy Days
Laverne & Shirley
Little House On The Praire

Little House on the Prairie
Three's Company

Melrose Place. All but two main cast members left Melrose Place at the end of season 5 so they brought in all these newbies in season 6 to help fill the void. It didn't work since FOX had to pull the show in the middle of the season and burn off the episodes with a final season the next year. Not even Heather Locklear could save that train wreck. Melrose Place proved if you're going to lose all of your cast in one season after being on for quite a while, just end the show. No new character can replace an established character fans grew to like for so many years.

Saved By The Bell The New Class. Only Belding lasted the entire series run. None of the kids from season 1 lasted long.

Once Upon A Time had a reboot in their last season with only two of the original cast members from the first season. Still it wasn't that bad though. I was expecting worst.

The Lucy Show had only Lucy that stayed the entire six seasons. Viv and the kids left at the end of season 3.

I used to watch the soap opera Days of Our Lives in the late 1980’s when I went to college and after college lost interest and never saw it again until 2018 or so. I live outside the USA and was home visiting switching the channels and notice Days of Our Lives was to begin and I decided to watch it for a moment.

I was shocked on how many of the players are still on the show thirty years later. Patch and Kayla are still there. A Martinez is also there. Amazingly John Aniston at 87 years old is still on the show. Aniston played a “bad guy” named Victor who was a mafia boss and the “good guys” (Roman, Patch etc.) were training to nail Victor in his evil deeds. That was a big fail because Victor is still around.

But it is also true that Soap Operas employ new actors for established characters after the previous actor leaves (or is fired). Soaps usually have actors who have been there for decades (the longest tenured actor of Days is a Suzanne Rogers who has been on the show since 1973) and young actors who come and go. The lifers are there because it is a steady paycheck and the newbies are there for experience.

Maybe someone else, but there were three or four players on there from thirty plus years ago

"NYPD Blue" - The only original cast member who stayed for the entire duration of the show was Dennis Franz. Everyone else either quit the show or got fired. That was one show whose cast kept changing - changing faster than bed sheets.

"Baywatch" - That's another show whose cast kept changing, and changing faster than bed sheets. For years, the only original cast member who stayed on was David Hasselhoff, with everyone else leaving the show left right and center. Even Hasselhoff himself left the show, leaving no original cast members left.

"Dallas" - When this show ended, nearly all of the original cast was gone. The only original cast members who stayed on the show until the end were Patrick Duffy and Larry Hagman.

"Dynasty" - The only two original cast members who stayed on the series were John Forsythe, and John James, I think.

"Knots Landing" - This show ended with most of its original cast members gone, with only Michele Lee and Ted Shackleford remaining, and perhaps Joan Van Ark - if you count her (she was absent from the final season, and only returning in the two-hour series finale).

"Fame" - Another show that ended with most of its original cast gone. The only four original cast members remaining were Debbie Allen, Albert Hague, Gene Anthony Ray, and Carlo Imperato.

"The Doris Day Show" - I read that the child actors who played Doris' on-screen sons were fired, with their characters written out with no explanation. According to other things I read, most all the other cast members either quit the show or were fired.

I thought of Doris Day too - she was the only survivor from season one still standing at the end

Mama’s Family started with Vicki Lawrence, Ken Berry, Dorothy Lyman, Karen Argroud, Eric Brown, and Rue McClanahan and ended with Vicki Lawrence, Ken Berry, Dorothy Lyman, Allan Kayser, and Beverly Archer.

Saved By The Bell The New Class. Only Belding lasted the entire series run. None of the kids from season 1 lasted long.

And if we're to include the Good Morning, Miss Bliss episodes as they were added to the original Saved By The Bell reruns the only characters to stay were Zack Morris; Screech; Lisa Turtle and Mr. Belding.

Barney Miller

Jag had only David James Elliot and Patrick Labyorteaux at the end of the 10th season. Patrick only appeared in the pilot in season 1, but became a regular in season 2.

Sliders - Only Cleavant Derricks of the original four Sliders made it to the end of the series. John Rhys-Davies left towards the end of season three. Sabrina Lloyd didn't return for season four. Jerry O'Connell didn't return for season five (the final season).

You Can't Do That On Television - Only Les Lye playing the producer of the show remained for the full run. All the original kids were eventually replaced.

sliders

You Can't Do That On Television - Only Les Lye playing the producer of the show remained for the full run. All the original kids were eventually replaced.

Speaking of Nickelodeon sketch shows, Josh Server was the only original cast member of All That to remain during its entire first run/first six seasons (1994-2000).

Melrose Place. All but two main cast members left Melrose Place at the end of season 5 so they brought in all these newbies in season 6 to help fill the void. It didn't work since FOX had to pull the show in the middle of the season and burn off the episodes with a final season the next year. Not even Heather Locklear could save that train wreck. Melrose Place proved if you're going to lose all of your cast in one season after being on for quite a while, just end the show. No new character can replace an established character fans grew to like for so many years.

Saved By The Bell The New Class. Only Belding lasted the entire series run. None of the kids from season 1 lasted long.

Once Upon A Time had a reboot in their last season with only two of the original cast members from the first season. Still it wasn't that bad though. I was expecting worst.

The Lucy Show had only Lucy that stayed the entire six seasons. Viv and the kids left at the end of season 3.

Melrose Place's parent show, Beverly Hills, 90210 ended its run in 2000, with only Kelly, Steve, Donna, and David being there from the beginning a decade prior. All of the Walshes (the last being Brandon) were long gone by that point. And Kelly was for all intents and purposes, the defacto lead character (she had already replaced Brenda as the female lead for many years leading up to that point) given that Jennie Garth became the first billed star after Jason Priestley departed.

I'm kind of surprised that Hazel hasn't been mentioned yet as Don DeFore and Whitney Blake were infamously dropped upon the show's move to CBS for its final season. This left Shirley Booth and Bobby Buntrock as the only original cast members left.

Yes, on the mention of My Three Sons in a previous post, after 12 seasons only Fred MacMurray and Stanley Livingston were left.

Similar situation with another old show, Make Room for Daddy/Danny Thomas Show. After 11 seasons and 7 or 8 different apartments, only Danny Thomas and Rusty Hamer remained standing.

Night Court only had three original cast members last from the series premiere to the series finale: Harry Anderson, Richard Moll and John Larroquette.

ER, which was on for 15 seasons, lost its final original cast member (Noah Wyle) at the end of season 11. Every original cast member from season one guest-starred at least once during the final season, though.

First season cast (1994-1995), in order of exit:
Sherry Stringfield (left season 3)
George Clooney (left season 5)
Julianna Margulies (left season 6)
Eriq LaSalle (left season 8)
Anthony Edwards (left season 8)
Noah Wyle (left season 11)

Sherry Stringfield did return during season 8, only to leave again at the beginning of season 12.

15th and final season cast (2008-2009), in order of joining:
Parminder Nagra (joined season 10)
Linda Cardellini (joined season 10)
Scott Grimes (joined season 12, recurring seasons 10-11)
John Stamos (joined season 13, recurring season 12)
David Lyons (joined season 15, recurring season 14)
Angela Bassett (joined season 15)

Charlie's Angels ended with only Jaclyn Smith and David Doyle still being around from the first season. Cheryl Ladd is the second longest tenured cast member, but she came in the second season after her on-screen sister, Farrah Fawcett left. If you want to count daytime soap operas, then The Bold and the Beautiful, which debuted in 1987, has Katherine Kelly Lang (Brooke Logan) and John McCook (Eric Forrester) to still be on the show today.

It's a Living ended its run in 1989 with only Barrie Youngfellow, Gail Edwards, Marian Mercer, and Paul Kreppel to still being on the show dating back to its debut on ABC in 1980. Another network to syndicated '80s sitcom, Charles in Charge only had Scott Baio and Willie Aimes to be on all five seasons.