TV Tropes refers to it as the "Other Darrin" trope: mid-show, a new actor replaces another actor in an already established role on a TV series.
Let's catalog a few of these here for DC-related shows.
ADVENTURES OF SUPERMAN: Noel Neill replaced Phyllis Coates as Lois Lane.
BATMAN: Eartha Kitt replaced Julie Newmar as Catwoman.
WONDER WOMAN: Not sure if this counts, but Lynda Carter replaced Cathy Lee Crosby as Wonder Woman. I don't know if the TV movie that Ms. Crosby appeared in was meant to be a pilot for the series that eventually starred Ms. Carter in the title role.
SHAZAM!: John Davey replaced Jackson Bostwick as Captain Marvel.
THE FLASH (2014/2015 series): The Linda Park character is slated to return, and she will NOT be reprised by the actress who briefly portrayed her in the premiere episode.
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