why did two different actresses play her?

Pamelyn Ferdin was the original Edna Unger but her character only appeared in a few episodes. She was cast as Paul Lynde's daughter in The Paul Lynde Show and because she would be appearing in every episode she and her parents thought it was a better opportunity for her. The Paul Lynde Show only lasted one season. Both shows were on ABC and were filmed in the same studio so occasionally Tony Randall and Pamelyn would run into each other and Tony would tease her that she picked Paul Lynde over him to be her father. Doney Oatman, who played one of Edna's friends in an earlier episode, took over as Edna but also only appeared in a few episodes as children weren't generally featured in the show. Pamelyn also did the voice of Lucy in many of the Peanuts cartoon specials.

I first saw Pamela Ferdyn in the Lassie TV Series in the early 1970's.
It was in Lassie's final 2 seasons 71-73. She wasn't on at the beginning, but when she joined she as on regularly.
I always assumed that was why she wasn't on the OC anymore.

Why were their 2 Leonard's?