When TV’s leading men and women leave

The heart of groundbreaking drama “ER” continued to beat after the movie-bound George Clooney pulled the plug on his heroic character, Dr. Doug Ross. The laughs kept coming even though Shelley Long’s Diane Chambers stopped slinging cocktails at " Cheers,” and “Star Trek’s” USS Enterprise stayed aloft for decades after William Shatner vacated the captain’s chair.

But “American Idol” without acerbic, tough-love judge Simon Cowell, “Larry King Live” sans Larry King, and “The Office” minus its clueless boss, Steve Carell as the always inappropriate Michael Scott — can the franchises thrive, even survive, without them?

Long-running hits are destined to go through some personnel shifts during their time on the air. Just look at the revolving door of talent on seminal series such as " MASH,” " NYPD Blue” and “Law & Order.” Fans might be forgiven, though, for having trouble envisioning “Idol,” “Larry King Live” and “The Office” without their leading men.

Fox is shaking up the 10th season of “Idol,” bringing back producer Nigel Lythgoe and assembling a judges’ panel that still has more than a few question marks, but may include Jennifer Lopez and Steven Tyler. NBC’s staying mum on whether it will promote from within or recruit a new Dunder Mifflin regional manager for “The Office.” Ratings-challenged CNN is in talks with British TV personality Piers Morgan to step into Larry King’s nightly interview show, potentially adding the “America’s Got Talent” judge to a reworked prime-time lineup that will include controversial ex- New York governor Eliot Spitzer.

Major cast changes don’t have to be a death knell — “Valerie” became “The Hogan Family” and survived without its namesake, Valerie Harper; “Spin City” thrived after Charlie Sheen stepped in for Michael J. Fox.

“It can be daunting, but it gives the writers an opportunity to shake up the dynamic and tell new stories,” said Jonathan Littman, executive producer of the “CSI” franchise and president of Jerry Bruckheimer Television. “If you’re lucky enough to have a show that lasts six or seven years, it’s inevitable that you’ll have change.”

" CSI: NY,” in fact, recently lost Melina Kanakaredes, who’s starred in the show since its launch in 2004 but wanted to move on to other projects; Sela Ward will join for the fall season.

“It’s good to have a stable cast because audiences get invested in those actors,” Littman said. “But viewers come back, ultimately, for the stories.”

Having a sizable ensemble cast may soften the blow, particularly when the show is highly formatted like the ever-popular crime-and-punishment procedurals. It’s tougher when a series is built around a character or star and is closely identified with that person, as is often the case with reality shows and comedies.

“In that situation, the networks usually want a big-name replacement, someone they can promote like crazy,” said Bill Lawrence, creator of ABC’s “Cougar Town,” who’s gone through numerous cast changes at previous sitcoms “Scrubs” and “Spin City.” “That’s a burden for producers who need to find a person who’ll work creatively and get the network excited at the same time.”

When Carell clocks out of “The Office” at the end of this season, at least one industry blogger thinks the show won’t recover.

“The original idea was to do a mockumentary about office life with the boss from hell,” said Jace Lacob, founder of the Televisionary blog. “If you lose him, you’ve lost the element that’s compelling and sustainable and relatable.”

Lacob, a former fan who said he’s found the single-camera comedy “unwatchable” for a few seasons, believes Carell’s farewell should be the series ending too.

NBC, whose executives wouldn’t comment, don’t agree, given that “The Office” rates highest on its Thursday night comedy lineup and attracts a well-heeled audience that’s a boon for advertising. Co-executive producer and costar Mindy Kaling has already thrown her support behind Rainn Wilson, whose beet farmer-sales dynamo Dwight Schrute could succeed, if not exceed, Michael Scott in the comedy of embarrassment.

Producers filling a high-profile vacancy should avoid trying to replicate the actor or star who just left, say industry watchers. That is, adding another sarcastic Brit to “American Idol” would be a mistake, just as a Carell clone wouldn’t work on “The Office.”

Morgan, on the other hand, may struggle because he’s so unlike Larry King, TV historian Tim Brooks said. He’s little known in the U.S. and not apt to draw the kind of “big catch” mega-watt political, media, sports and entertainment figures that were King’s stock in trade.

Even if “Idol” loads up on famous faces, it will need someone with an edge to dole out real criticism, Brooks said, and could lose viewers without the love-to-hate-him Cowell. Ratings have already slipped double digits, though “Idol” is still the No. 1 show on broadcast TV, averaging 24.9 million viewers. It’s considered so important to advertisers that they’re reportedly paying as much as $650,000 for a 30-second spot in the show’s next season, more than they forked over during Cowell’s last lap.

Fans won’t necessarily tune out just because there’s a changing of the guard, even though they may protest loudly, as they did for each new “Trek” captain when “they were so upset you’d have thought the Borg were attacking,” Brooks said. “But if it’s handled well and made organic to the story line, most fans will give it a chance.”