Looking at the information about the character on Wikipedia she was played by four different actors:
Donna Bullock, Ann Ford, Nancy Bleier &
Jeanna Michaels
Now it's possible the Producers wanted a long term actor when the show became a hit, I'm only speculating here.
Either that or perhaps the actors left of their own accord as being a secretary on a TV show like Dallas is only really a background character.
My last theory is that Jeanna Michaels was quite similar looking to Linda Gray which may or may not have something to do with why she left or was maybe possibly replaced by the Producers.
I wonder what really happened why the character was replaced? Of course it just could be because she was played by four different actors that they wanted a different character instead.
