A character replacement, also known as an Impostor!Sue, a Replacement!Sue, or a Doppelgänger!Sue, is what you have when a canon character is so badly out of character that mere possession or Suefluence no longer explains their behavior. When a character comes close to or reaches 100% OOC, they split into two characters: one character replacement, and one canon character. The canon falls or is forced into a plothole; the replacement takes over the character's role.
A character replacement happens when everything that makes the canon character who they are is tossed out the window while the story continues to insist they're the same person. As in cases of possession, the character will exhibit sudden, unexplained, severe changes in appearance and/or behavior: a canonically kindhearted character may abruptly develop a temper and turn on her friends and family; a canonically cruel or capricious character may act like a woobie who weeps at the thought of his loved ones. The greater the alteration from canon and the poorer the explanation for the changes, the more likely it is that the character has been replaced.
The signs of a character replacement are very similar to those of possession by a Sue-wraith. Since mistaking a case of possession for a replacement can result in the unjust slaying of canon character who only needs to be exorcised, it is important to observe the character closely for a period of time to determine which is the case.
A name-change usually denotes a replacement. As does a complete moral transplant: for instance, Harry Potter driven by guilt and ANGST to wander around killing Death Eaters and capslocking about it is probably possessed; Harry Potter killing Death Eaters for fun and profit and not angsting OR capslocking about it is mostly likely a replacement. Dark! versions of heroes and good! or conflicted! versions of villains are not necessarily replacements — the key is to read how the author has justified it. If they've genuinely been extrapolating from canonical clues, hints and behaviours, then it's probably a case of possession. If they've completely changed the character's motivations, then you're most likely dealing with a replacement.
However, not all character replacements are Sues. Some may be removed from the continuum by means of recruitment.
When a character is replaced, the original is locked up in a plothole to prevent them from interfering with the fic. The duty of the agents is then to find the original and free them. Kidnapped canons can often be found in canonical prisons or other hard-to-escape places. They may require neuralyzation and/or a trip to the Department of Fictional Psychology, depending on what they may have seen or experienced.
A problem arises with Stus that try to impersonate Captain Jack Harkness. Jack can't stay dead, and for some reason his impostors have this characteristic, too. On their one encounter with an Impostor!Jack, Agents Emma and Tasmin decided to dispose of the impostor by taking him into Greek mythology and chaining him up next to Tantalos.
- "To Know Where You Are Going" (Lord of the Rings), Agents Jay and Acacia (DMS)
- Featuring Sue!Arwen and Pedo!Elrond.
- "Impostor" (Pirates of the Caribbean), Agents Mara and Isaiah (DTE) with Quen (DoP)
- "Family Ties" (Harry Potter), Agents Supernumerary and Ilraen (DIC)
- Featuring a weepy Snape doppelgänger, Sithchean Bran.
- "Seven Eighths" (Doctor Who), Agents Tawaki Penguin and Dustin O'Grady (DTO)
- In which the Seventh Doctor is replaced by a doppelgänger with Captain Jack Harkness' mind.
- "The Dark Side" (World of Warcraft x Fullmetal Alchemist), Agents Supernumerary and Ilraen (DIC) with Agents Barid and Brightbeard (APD - Warcraft)
- In which a case of possession is mistaken for a character replacement.
- Numerous characters in That Series, Cregga and Mariel most obviously.
- Every single canon character in "For Your Eyes Only."
- Most of the characters in "My Immortal."