When was that? The latest season posted a month ago. It was cancelled after 2 seasons but was picked up for 2 more on Peacock. I don’t see any info if it’s coming back for a 5th season.
Gotta confess, it slipped by me that the show was picked up for additional seasons streaming. It looks like it was at a reduced episode order, so I guess Howerton was better able to split duties.
Dennis Franz? He was killed off in Hill Street Blues as Dirty Sal Benedetto, and moved on to Bay City Blues. After that flopped, he returned to Hill Street Blues as Norman Buntz.
In Bonanza, eldest son Adam Cartwright (Pernell Roberts) left the show fairly early, while it was the $1 rated TV show. The story line was that he had gone ‘back east’ to go to college. But that left it open for him to be written back into the series if they ever wanted to (and the actor had agreed – he had not like the show quality, considering it formulaic). The writers even tossed in occasional lines where his family referred to him & his activities, thus keeping it open for him to return, or even do a guest appearance as a ‘visit home’.
Roberts went back to live theatre, in touring companies and then on Broadway. He made guest appearances in lots of TV shows, also, and then starred again as the lead in Trapper John, MD.
Kam Tong played Hey Boy on Have Gun Will Travel. Tong left for a bigger role on another show, Mr. Garlund. His character was replaced by Hey Girl, played by Lisa Lu. When Mr. Garlund was cancelled, Tong returned to Have Gun Will Travel.
Parker Young played Ryan Shay, Tessa Altman’s bf in Suburgatory, then was written out when he left to star in Enlisted. Enlisted got cancelled after one season, so Young’s character returned to the last couple of episodes of Suburgatory to marry Tessa.
Michael Shanks played Dr. Daniel Jackson on Stargate SG-1. He left for a season, and his character was said to have “ascended.” A character named Jonas Quinn joined the team. Jackson broke the rules and and became mortal again and rejoined the team, and Quinn left.
In a twist on the OP: Gates McFadden was fired from ST:TNG after the first season. Her replacement crashed and burned*, and she was begged to return.
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- Crashed and burned, fell down a turbolift shaft. Tamayto, tamahto.
And then Quinn came back for one episode in the following season.
Just thought of another example from The Dukes of Hazzard. No, not Bo and Luke’s short-lived replacement by Coy and Vance – that was a pay dispute with Wopat and Schneider, not them leaving for another show.
But Sonny Schroyer (AKA Deputy Enos) left the show after the third season for his own spinoff (called, appropriately enough, Enos). That show only lasted 17 episodes, and he’d specifically had it written into his contact that if Enos didn’t make it at least one full season he had to be returned to Dukes. Which he was for season five. In the meantime the show had brought on “Deputy Cletus” who was functionally the same character. For a time both characters continued to appear on the show before Cletus “went on vacation” and was never heard from again.
The final episode of Homicide: Life on the Street (Homicide: the Movie) brought back almost every actor who had been in the main cast – including several characters who died.
The 30 Rock/Whitney/30 Rock one was the one I thought of.
Update: as I suspected the new show was cancelled.
And the actress has returned to Chicago Med for at least the 2 most recent episodes this season 
I’m interested to see if it’ll only be these two episodes or if it’s going to be a more permanent return.
In Magnum PI, Jeff MacKay’s character Lt “Mac” McReynolds was killed spectacularly so the actor could be in Bellisario’s new show Tales of the Gold Monkey. Which lasted all of almost one season. So they brought back the actor to Magnum as Jim “Mac” Bonnick, a character who bore a remarkable and oft-remarked resemblance to the late Lieutenant.
Ya know, even as a kid, obsessed with Ferraris, I watched that show and thought to myself, “Oh come-fuckin-on! How stupid do you think we are???”
Sadly, writing for TV shows has been on a steady decline since then.
The worst spin off disaster has to be The Ropers.
IIRC it basically wrecked Norman Fell and Audra Lindley’s careers.
They couldn’t go back to Threes Company because Don Knotts had replaced them.
Stephen Furst never officially left Babylon 5, but his character, Vir Cotto, was moved mostly offstage to the Minbar homeworld, so that he could do another show. When that show failed, Vir was found to have blotted his copybook (by helping refugees—oh! the horror of it!), and was brought back to his old job on Babylon 5.
Also, Michael O’Hare had a nervous breakdown from bearing the weight of being a star (“If I screw up, hundreds of people will lose their jobs!”) and had to leave the role of Jeffrey Sinclair. But Sinclair was kept alive, and his off-screen adventures continued in paperbacks and comics, until O’Hare was well enough to come back for a spectacular two-part episode that radically altered the entire show.
And Johnny Sekka had to leave the role of Dr. Benjamin Kyle after the pilot because of a medical condition that kept the actor from getting performance insurance, but Dr. Kyle was kept alive on Earth in case the actor ever became available again, and, at the end of the show, it was Dr. Kyle who was (still off-screen) responsible for his replacement, Dr. Stephen Franklin (Richard Biggs), leaving Babylon 5 to replace him again on Earth.
The Andy Griffith show lost Don Knotts and it was never as good.
Don said later that Andy had insisted he would only do 5 seasons. So Don sought out other work. Universal offered him a 5 movie deal. Andy changed his mind and came back to the Griffith show for season 6. By then Don couldn’t walk away from the chance to star in movies.
I think season 6 could have been much better with Don. The show was starting to run out of fresh ideas. It really shouldn’t have went beyond season 6.