Here's another one which Helen Cutter of 'Primeval' must answer for:
According to the IMDb, "In the pilot [of 'Father Knows Best'], Ellen Drew played the mother (replaced in the series by Jane Wyatt) and Sally Fraser played the eldest daughter (played in the series by Elinor Donahue.)"
For those of you who never saw 'Primeval', Helen Cutter journeyed back in Time through an anomaly and altered the course of History so drastically that it many times changed the chromosomal make-up of other TV characters, or - in the case of William Talbot MacNeil (President of the United States in the early 1970s as seen on 'Voyage To The Bottom Of The Sea') - wiped them out entirely.
Margaret Anderson's DNA was altered but her life basically remained the same. She still married Jim Anderson of Springfield and raised three children - Betty, Bud, and Kathy. With Bud and Kathy the combination of DNA from both their parents remained the same. However, with their first-born, Betty, there was just enough of a difference so that she no longer looked the same as she did in the original timeline.
We don't use this splainin for every recastaway. For example, the difference in Rob Petrie et al from the 'Head Of The Family' pilot to 'The Dick Van Dyke Show' can be attributed to Alan Brady buying the rights to Rob's autobiography and adapting it to become a sitcom starring himself as Rob. (As such, the 'Head Of The Family' pilot is the actual TV show within a TV show and takes place after 'The Dick Van Dyke Show' on the Toobworld Timeline even though it was filmed first.)