We went through two Bobbi Jo's and three Billy Jo's. Who could have stepped in as Betty Jo if Linda Kay Henning had left for some reason? It look like she was 19 when the show started in 1963. It would have to be someone who was working in say, 1965 or 1966 when they were replacing the other roles, but not so famous that she couldn't step into an ensemble. Is red hair necessary?

One person I'd look at would be Brooke Bundy, who was guesting on everything and had a pretty good gift for comedy. She was blonde, though, and I don't know if she ever played a redhead.

Finding a young redhead at that time who was experienced in comedy would've been a challenge. I believe in the later seasons, Linda wore a wig (compare with her natural hair on her 1970 Family Affair guest role), so maybe the producers could've gone that route for a Betty Jo replacement as well.

Brunettes like Karen Valentine, Reba Waters (older), Lori Martin (younger), and Shelley Fabares would have made good replacements for Bobbi Jo, if needed at some point. More blondes potentially as Billie: Candy Moore, Pamela Austin, or Cheryl Holdridge.