Final Season Casting

Being involved with a show for years, while rewarding, can be tiresome for actors. They might want to move on to different things, or the reduced budget isn't enough to pay their salary, or they signed up to something else before the show was unexpectedly Un Cancelled, or something else happens in their life that forces them to leave. Long story short, at some point they Opt Out before the show ends and their characters are written off. And more often than not, the show's ratings take a toll, as a part of its fanbase loses interest.

Now imagine what happens to the ratings if the actor in question was the main character. Or if more than one actor quit at the same time. Or both. You're stuck with a show that no longer has a protagonist and/or half of its cast and is steadily losing its viewership. Either you try to bring new characters to fill the void, rewriting the scripts accordingly, or you can try to function with what you have left. Either way, even if you do it right, the show just won't be the same now and the chances to be renewed for another season after this are slim to none.

In a nutshell, a Final Season Casting is a rather common phenomenon when a show experiences unusual and significant cast changes for its final season. Expect core members of the show's central Five-Man Band or even the protagonist to leave. Sometimes Suspiciously Similar Substitutes or new characters without much back story take over, sometimes the show continues with a reduced cast. The dynamic between the cast is very likely to change and if the show hasn't jumped the shark yet, it most probably will.

This trope makes no distinction between the seasons that weren't supposed to conclude the show, but turned out to be the final seasons because of low ratings partially caused by the Final Season Casting, and the ones that were supposed to conclude the show, and the Final Season Casting was either willingly invoked or was a mere coincidence.

A very common effect of this trope is They Changed It, Now It Sucks and common attribute of a Post Script Season. See also Jumped the Shark, Replacement Scrappy.

Live Action TV

 * Vaughn and Weiss were absent for most of season 5 of Alias. Two rather generic characters, Rachel Gibson and Thomas Grace, were introduced instead.
 * The producers were actually hoping for a spinoff with Rachel and Thomas, plus another new recurring character Renee Reienne, but when the network didn't oblige them, Thomas and Renee were both rather suddenly killed off, with very little effort to provide resolution to their own story arcs.
 * Ally McBeal season 5 ; horrible culling of many original cast members and replacement with far less interesting characters. The final scene of final episode still featured all the old cast and none of the new. All in all, rather contrived.
 * Angel pulled it off for its last season. As James Marsters said about his involvement with Buffy the Vampire Slayer: "Spike was supposed to be the new Cordelia". In the final season of Angel, he replaces her for the second time, even taking her place in the opening credits. Also Lilah, a prominent recurring character since season 1, is replaced by Eve.
 * The actress who played Susan Ivanova left Babylon 5 before the fifth (and final) season, necessitating the arrival of her Suspiciously Similar Substitute, Elizabeth Lochley.
 * Ivanova and Marcus both left at the same time (although Marcus was killed off before the writer realised the show was being renewed for another season - and he later said that had he known that Claudia Christian would be leaving, he would have killed her instead of Marcus). The loss of these two characters at once adversely affected the cast dynamic.
 * Daryll, who was in Charmed since day one, was absent from the final season. Leo didn't appear much neither. Billie was introduced. No comments.
 * Coupling had this going with one of the guys (Jeff) being replaced, with the new guy being (poorly) photoshopped into the cast photo on the cover of the series DVD collection.
 * Family Matters also did this, but there was also a Channel Hop that further complicated matters.
 * Scrubs had this happen in the final season. JD, Elliot, Carla, and the Janitor were all written off.
 * In the final season of Sliders, the main character Quinn was replaced by an alternate universe version who looked nothing like him, but was apparently melded with his personality, and Colin was lost forever, and replaced by someone else from the same universe as the second Quinn. Thus the show lost both the protagonist and half of the cast for its final season. Also,
 * In the final season of Home Improvement, Randy was written out of the show by having him spend a year in Latin America. He wasn't replaced with anyone, but other characters were given increased roles and/or new dynamics to fill the absence.
 * Partial example in Spin City. When Michael J. Fox could no longer do the show, he was replaced by Charlie Sheen. The characters of Nikki, Janelle, James, and Stacy also disappeared. But the show lasted for two more seasons with this cast, not one.
 * Heather Locklear, who had come on board a season before, shared top billing with Sheen. The remaining cast members all got their own title cards.
 * In Star Trek: Deep Space Nine, Jadzia Dax was replaced by Ezri Dax for the final season when Terry Farrell left for Becker. It was a particularly curious case as her departure was a real surprise to everyone involved, as Farrell admitted to loving the show and the character but was just interested in expanding her career. She later regretted the decision because while she enjoyed her new character, Dax was a "super hero." To the writings credit, Ezri managed to fit into the series well and wasn't just Jadzia with a new face (although that would be justified)
 * Eric and Kelso were no longer in the cast of That 70s Show during the final season, and were replaced by Randy. Both returned for the Series Finale.
 * In the final season of News Radio, Bill was replaced by Max after Phil Hartman's death. Also, Walt, a character introduced late the previous season, disappeared without a trace.
 * Catherine left in season 4 as well, and wasn't replaced, meaning that there was a 25% turnover from one season to the next. It was noticeable and was probably the last straw for the show which, despite it's quality, had been hanging by a thread for most of its existence.
 * The X-Files in its last two seasons. The eighth season had Mulder missing for over half its run, and partnered Scully with the new character of John Doggett; the ninth season didn't feature Mulder at all until the two-part Grand Finale, significantly reduced Scully's screen time and made Doggett and Monica Reyes the two agents attached to the X-files.
 * Inverted with Law and Order: Criminal Intent. The 10th (and final) season brought the return of Vincent D'Onofrio to the role of Bobby Goren, and Kathryn Erbe (Alex Eames) back as well.
 * Welcome Back, Kotter had a big problem with this during its last season. Both the nominal star (Mr. Kotter) and the most popular character (Barbarino) all but quit the show. Mrs. Kotter, of all people, took up the slack for Mr. Kotter, and the show brought in a replacement for Barbarino named Beau. Many viewers found Beau to be a pale imitation of Barbarino, and were not impressed with the "twist" of Beau being a Southerner.
 * Laverne and Shirley is an infamous example. Shirley left and the show continued without her, strangely keeping the title intact. The show compensated by bringing on weekly guest stars to assist Laverne in her schemes.
 * Robin Hood deliberately killed off for cheap shock value in series two, and the actors playing Will and Djaq left as well. Furthermore, Keith Allen could only sign on for half of series three, leading to the absence of Vasey for several episodes. To compensate for this loss of the original cast, the show introduced the characters of Isabella, Kate, Tuck and Prince John - Prince John being the only one who went down at all well with fans. Later on, Robin and Guy's long-lost half-brother Archer appeared and was equally unpopular. The show ended with the deaths of , with Sam Troughton (Much) also quitting. A planned fourth series was thankfully not commissioned, but it would have left Little John as the only remaining original cast member.
 * The Practice replaced half its cast in the final season, at least the replacements were fairly interesting.
 * The cast shake-up was mainly a way to shoehorn in a Poorly-Disguised Pilot for Boston Legal. Since the spun-off show was so good, we'll give David E. Kelly a pass.
 * Stargate SG-1 retired Jack O'Neill and replaced him with Cameron Mitchell for the final two seasons. Vala Mal Doran was also added to the cast, although no one else left.
 * Season 9's beginning looked to be this, though, with the arrival of newcomers Mitchell and General Landry, and Mitchell's attempt to 'get the band back together' resulting in an SG-1 consisting of him, Daniel, Vala, and Teal'c. Those familiar with SG-1 looking at the largely new cast with a "who the hell are these guys?" reaction was a Running Gag. The full return of Carter (who had been written out of the first several episodes due to Amanda Tapping's real life pregnancy) and an increase in O'Neill guest spots would come later.
 * Ivan Dixon left the cast of Hogan's Heroes when his contract expired, leading to Newkirk taking over some of Kinch's duties and Kenneth Washington playing the new communications officer, Sgt. Baker.
 * Saved by the Bell presents a curious case. The intended final episodes were filmed with the entire regular cast, including a Grand Finale where the teens graduated. Problem is, afterwards, more episodes were ordered, but no one was under contract. Most returned, but Tiffani-Amber Thiessen and Elizabeth Berkley chose not to. Leanna Creel was hired to play new character Tori and her tenure was interspersed with the previously produced episodes.
 * Subverted - twice - in Blakes Seven. Gareth Thomas who played Blake left at the end of the second season, not the last, and wasn't replaced. Despite there being no change in title, there was no Blake for the next two seasons  This caused much ridicule among critics and some fans at the time, but can be justified in that although Blake was gone, the rest of the Liberator's crew were still products of his rebellion and, in a sense, still his creations, carrying on his fight.
 * Main character Joel never quite disappeared from Northern Exposure, but in the last season his role was drastically reduced and the town got a different doctor.
 * Prior to the filming of season 4 of Hannah Montana, Mitchel Musso (who played Oliver Oken) left the series to star in Pair of Kings and work on his music. He and his character appeared only in a few episodes as a guest, including the series finale. He was replaced by Australian actress Tammin Sursok, who played Jackson's bikini model girlfriend, Sienna.
 * Multiple replacements in the final season of the original Hawaii Five-O: Dan-O (James MacArthur) and Chin Ho (Kam Fong) were replaced by Kimo (William Smith), Lori (Sharon Farrell) and Truck (Moe Keale). Although the series had run its course over 12 seasons, the massive recasting probably put the final nail in its coffin. In the case of Chin Ho, by the actor's request, the character didn't just disappear but his murder was made integral to the episode's plot.
 * Earth: Final Conflict was doing this LONG before its final season. The show's producers were infamous for immediately firing any actor who asked for a raise. Only one character (a villain who largely functioned as The Dragon for the current Big Bad) was a regular in all five seasons.
 * House. Lisa Edelstein left after season 7, so Cuddy was written out and Foreman (Omar Epps) became the dean of medicine at Princeton-Plainsboro. Also, the characters of Dr. Chi Park (Charlyne Yi) and Dr. Jessica Adams (Odette Annable) were added to House's team.