Much has been made in recent weeks about Charlie Sheen's massive paychecks and even bigger ego, something that won't be a problem any more for Warner Bros. Television and Two and a Half Men.But if the hit CBS sitcom really does go forward next season without Sheen, as apparently is the plan, there is still the pesky matter involving the size of the troubled actor's shoes. Shoes that are going to be mighty hard to fill.The same is true for whoever gets selected to replace likable Steve Carell once he parts ways, on much friendlier terms, with The Office later this season.It's entirely possible that these shows can remain hits without their longtime leading men. The replacement game is practically an annual TV ritual, with classic characters and beloved stars coming and going with revolving-door regularity. Yet it's hardly a sure thing that the two shows will survive.History of animosityWhile most splits are amicable (such as George Clooney's departure from ER, William Petersen's farewell from CSI and David Duchovny's reduced presence during the final seasons of The X-Files), sometimes, as with the feud that led to Sheen's firing this month, it gets very ugly and goes very public.Suzanne Somers knows what that's like. The former Three's Company star took the hit 30 years ago in a salary dispute that "spun out of control almost before it ever really began," she says.When Three's Company began its fifth season, Somers asked for a hefty pay raise (from $30,000 an episode to $150,000, a fraction of the $1.2 million per episode that Sheen had recently been making). Not only was she rebuked by the network and studio, but "they chose to make an example of me," she says.Somers became persona non grata on the set and was denied contact with co-stars John Ritter and Joyce DeWitt. Her role was reduced to one minute per episode and, to film her segments, she was escorted by security guards to a separate soundstage. At the end of the season, she was pink-slipped."I was on 55 national magazine covers that first year of Three's Company," Somers recalls. "My life went from being totally anonymous to being so unbelievably public ... and then, almost as quickly, like in the blink of an eye, I had become blacklisted within the industry."Making matters even more painful was that, while Somers basically had to start over, the show continued to thrive with Priscilla Barnes brought in as Ritter and DeWitt's new on-screen roommate.The public humiliation was something Somers "wouldn't wish on my worst enemy," she says.Not so smooth movesOther stars have become show-business laughingstocks over their feuds with, and early departures from, hit shows.When David Caruso wanted to bolt after one season of overnight success on NYPD Blue (he was replaced by Jimmy Smits), he was allowed to leave but came off looking like an egomaniac -- and he paid for his hubris with a going-nowhere film career that included, fittingly, a movie titled Kiss of Death. Now the longtime leading man on CSI: Miami, a humbler Caruso has says he "learned my lesson."Farrah Fawcett didn't fare much better in the movies when she left Charlie's Angels after one season. It took many years for her to find her bearings as an actress, while replacement "Angel" Cheryl Ladd immediately fit in and the show ran four more years.Shannen Doherty, who is known for being a difficult presence on the set, was fired from two series, Beverly Hills 90210 (after four seasons and replaced by Tiffani Thiessen) and Charmed (after three, replaced by Rose McGowan). Both shows kept going strong for many years without her.McLean Stevenson ditched M*A*S*H after three seasons, thinking he should headline his own show. His subsequent star vehicle was a stinker called Hello, Larry, and he became an industry punch line as a result.When Valerie Harper waged a salary dispute with the producers of her sitcom Valerie, they killed off her character and brought in Sandy Duncan. The show, renamed The Hogan Family, ran four more years.And then there's the case of Shelley Long, who endured a "how dare she?" backlash for having the audacity to leave her Emmy-winning gig as the leading lady on Cheers.Actually, Long protests being lumped into the same category with Fawcett, Caruso and Stevenson.Here's why: "I had a contract for five years, and I finished my contract," says Long, who was replaced by Kirstie Alley. "Once I finished my contract, it was up to me: Did I want to stay or not? But some mean-spirited stuff started coming, some it from the Cheers people. Ted Danson has admitted that to me. And it came from fear, as most stupid things do."I guess they felt that the show wasn't going to be able to continue without me. I kept saying: 'Wait a minute. Look at all the talent here. That's not going to happen.' In actuality, me leaving gave the show a second life. But nobody figured that out until later. And by then, hurtful things had been said."Taking chancesIt's always something of a crapshoot how a key personnel change will affect a show. Sometimes it's a shot in the arm; other times, it's a huge and immediately obvious mistake.When Mission: Impossible brought in Peter Graves as the leader of the team in Season 2 (replacing Steven Hill), it was a great move. The show later jettisoned Martin Landau and Barbara Bain (replacing them with Leonard Nimoy and Lesley Ann Warren), which was not so smart.M*A*S*H endured frequent cast changes (Stevenson out, Harry Morgan in; Wayne Rogers out, Mike Farrell in; Larry Linville out, David Odgen Stiers in; Gary Burghoff out, expanded role for Jamie Farr). The comedy continued to thrive, but each change took the show farther away from its finest moments.When Law & Order brought in Jerry Orbach (for George Dzundza) and Jill Hennessy (for Richard Brooks), the show benefited greatly. Later moves, such as replacing Angie Harmon with Elisabeth Rohm and replacing Orbach with Dennis Farina, weren't so effective.Charlie's Angels broke even in the Ladd-for-Farrah exchange, but replacing Kate Jackson with Shelley Hack (and with Tanya Roberts a year after that) coincided with the show's fall from grace.When CSI: Miami ditched Kim Delaney midway through its first season (allowing more screen time for co-star Emily Procter), it was right for cast chemistry. But adding Eddie Cibrian essentially added nothing and the ultimate solution was to kill off his character.After Raymond Burr died in 1993, NBC showed questionable taste by making more Perry Mason mystery movies, with Paul Sorvino and Hal Holbrook handling the cases while Mason was "out of town."Even Sheen has been on the other side of the replacement game equation. When Michael J. Fox left Spin City after four seasons because of Parkinson's disease, Sheen took over as the male lead, extending the show's life two more seasons.One of the most creative character handoffs, meanwhile, occurred on Star Trek: Deep Space Nine. Terry Farrell played the humanoid host for an alien life form; when she bid farewell to the show, the alien inside her was transferred to a new host, played by Nicole deBoer.That's probably not a viable plot possibility for Two and a Half Men, even though everyone seems to be wondering what's gotten into Charlie Sheen.