Star Trek: TNG
Gates McFadden as Doctor Beverly Crusher replaced by Diana Muldaur as Doctor Katherine Pulaski
Situation: Gates McFadden played Dr.Beverly Crusher during the first season of Star Trek: The Next Generation, between the shows hiatus the producers had felt that the character wasn't working out as well as they had liked along with Gates clashing with producers over her characters development lead to her being fired before the start of Season 2. Diana Muldaur came in to fill the void of the ships doctor, however Diana reportedly faced backlash from some of the cast who felt a loyalty to Gates. Dianas character Dr.Pulaski did not catch on with viewers and coupled with Dianas dislike of working on the show lead her to leave after only 1 season. Cast and fans rallied for Gates to be brought back and the producers brought Beverly Crusher to the show in Season 3 where she stayed for the remainder of the series.
Mighty Morphin Power Rangers
Austin St. John/Thuy Trang/Walter Jones replaced by Steven Cardenas/Karan Ashley/John Bosch
Situation: Austin/Thuy/Walter played the Red/Yellow/Black Power Rangers for the 1 1/2 seasons of MMPR. With the show becoming the biggest kids show in the world, the cast attempted to get pay raises due to all the time and effort they were putting into the show but Saban was not willing to give the cast more money so in retaliation half the cast (A/T/W) decided to stop showing up for work in the middle of the season which lead production to get creative, at first the show used stand ins/voiceovers/reused footage to get around half of their cast not showing up. The show knew they would have to be written out somehow if they would not come back so they had Jason/Trini/Zack go off to participate in a World Peace Conference and lead to the first of many major cast shakeups which would help Saban show the cast that they are replaceable at any time. The show brought in 3 new characters to take up the Red/Yellow/Black Ranger mantles and they went on to star in the MMPR movie that was originally intended to include the original cast.
Beverly Hills 90210
Shannen Doherty as Brenda Walsh replaced by Tiffany Amber Theissen as Valerie Malone
Situation: Shannen Doherty played Brenda Walsh for the first 4 seasons of BH90210 but after reportedly becoming a problem behind the scenes with constantly showing up late and making life difficult for some of her cast members Brenda Walsh was shipped off to live in London while Shannen Doherty was fired from the show. Theissen came in for the shows 5th season playing Valerie Malone, cousin to Brenda and Brandon Walsh taking up Brendas leftovers by becoming rivals with Kelly Taylor and sleeping with Brendas ex boyfriend, Dylan McKay. Tiffany remained on the show up until the shows last season when she quit to do other acting opportunities.
Star Trek: Deep Space Nine
Terry Farrell as Jadiza Dax replaced by Nicole De Boer as Ezri Dax
Situation: Terry Farrell played Jadzia Dax for 6 of the 7 seasons of DS9. With her contract up at the end of Season 6, Terry attempted to renegotiate her contract to include fewer episodes as she was exhausted from the long hours but Rick Berman told her it was either all or nothing. Terry decided not to sign on and DS9 decided to kill Jadzia Dax. The show brought in Nicole De Boer for the shows final season as Ezri Dax, a character who ended up taking Jadzias symbiont and memories. Years later EP Ira Steven Behr was let in on the bts drama and he noted had he known, he wouldn't have killed off Jadzia and brought Terry part part time.
Star Trek: Voyager
Jennifer Lien as Kes replaced by Jeri Ryan as Seven of Nine
Situation: For 3 seasons, Jennifer Lein starred as Kes amongst the Voyager, at the end of the shows 3rd season the writers wanted to bring on a new character but due to budgetary constraints they would have to get rid of one of their original cast members. Originally the writers were going to kill off Garret Wang who played Harry Kim on the show but due to Wang surprisingly being named one of People Magazines 50 sexiest men which helped garner attention for the show, he got a reprieve and Kes was chosen as his replacement to write off the show. Jeri Ryan came on at the start of season 4 with the writers using Kes to help Seven of Nine remain her humanity before killing her off. This lead to tension on set between Jeri Ryan and Kate Mulegrow who played main character Captain Janeway. Jeri stayed on for the remainder of the shows run.
Shannen Doherty as Prue Halliwell replaced by Rose McGowan as Paige Matthews
Situation: Shannen Doherty who was famously replaced on 90210 by Aaron Spelling ended up reteaming with Spelling for a new series called Charmed that centered around 3 sisters who happened to be witches. During the shows 3rd season, tension was rising between Shannen and her co-ster Alyssa Milano who played her onscreen sister, Phoebe Halliwell. By the end of the season, producers decided that one of the actresses had to go, given Shannens past history and the fact that Prue had less storylines to wrap up, it was decided during the summer hiatus that Prue Halliwell would be killed off screen. Rose McGowan came in the 4th season as the long lost sister of the Charmed Ones. She inherited Prues telekinetic powers but with a slight change due to her being half whitelighter. McGowan went on to stay until the show ended in Season 8.
Katie Cassidy as Dinah Laurel Lance/Black Canary replaced by Juliana Harkavey as Dinah Drake/Black Canary
Situation: Katie spent 4 seasons of Arrow as her character developed into superhero Black Canary, at the beginning of the season the writers teased a big death coming that would leave someone from the show dying at the end of the season. While they attempted to maintain that everyone was up for grabs and that they were not sure who they would kill off, at the end of the season it was decided that Black Canary would bite the dust and Katie Cassidy left the show as Black Canary was said to not necessarily be needed in the show. In the middle of the shows 5th season the writers brought in Juliana Harkavey to play Dinah Drake and take up the mantle of Black Canary. In the comics, Dinah Drake is DLLs mother which caused confusion for some fans. While everyone bts claims there is no bad blood and KC was brought back a handful of times for flashbacks/fever dreams/doppelganger, fans have speculated that the inconsistent writing and sudden resurgence by the writers of how important Black Canary is to the Green Arrow mythos has lead fans to speculate that the writers did not enjoy DLL or possibly KC.
Nicole Behair as Lt. Abbi Mills replaced by Janina Gavankar as Agent Diana Thomas
Situation: Nicole joined Sleepy Hollow as one half of a dynamic duo with Tom Mison as Ichabod Crane, the first season enjoyed rave reviews and great ratings but by the time the 2nd season came around it was obvious things were not going well bts. Abbie Mills was put on the backbuner for Season 2 while Cranes past wife, Katrina Crane gained more screentime and Nicole was being shut out of things such a commentary for the show. After ratings plummeted and the show received backlash from fans, the 3rd season returned to focusing on Abbie partnership with Ichabod Crane but the damage had already been done. Nicole and Sleepy Hollow agreed to part ways at the end of the season with Abbie being killed off. Janina was brought in as Diana for the shows 4th season to become Icabod Cranes new partner. The shows ratings have fallen even further but with a 5th season in limbo.
2 1/2 Men: Charlie Sheen replaced by Ashton Kuther
Source: My memory/tv + 2 3
I might've missed plenty but these are all characters who left an impression on me.`