[laumars]

TV shows and movie franchises often replace cast. Sometimes with the new actor / actress playing an existing character.

* Sometimes it's done out of necessity so the changes are glossed over (Dumbledore changed characters due to Richard Harris sadly passing away, or when actors/actresses get fired from soap operas),

* sometimes it's made a feature (eg Doctor Who regenerations)

* and sometimes they'll just introduce a new leading character to replace the old one (eg the crime comedy "Death in Paradise" which has had 3 different leading inspectors as the previous actors have left the show).

Few people, if indeed anyone, are irreplaceable in their careers.