I know that the producers always had their sights set on Markie Post to play the public defender. But at the time, Post was under contract to do The Fall Guy. The official reasoning for why Ellen Foley was let go was because the relationship between Billy and Judge Stone, felt more like "brother and sister" instead of potential love interests.

Ellen Foley always seemed more scrappy, streetwise and quirky in comparison to more traditional glamour girls like Markie Post. Maybe I'm being cynical sounding, but I figure that producers felt that they needed a "hot babe" to lure the guys in (because I doubt that they would be into Selma Diamond) and Ellen Foley's Billie was too off-kilter.

https://media.giphy.com/media/8F3bXRrLu66OU00RX9/giphy.gif

I liked Ellen Foley and the Billie character. It does seem that she was just a placeholder until Markie Post was available though. I wonder if the producers told Ellen that she was temporary or not?

Shelley Hack was originally supposed to play the Christine Sullivan character.

http://shelleyhack.blogspot.com/2011/11/night-court.html

They don't call her Hack for nothing. Shelley couldn't act and had the Hollywood primadonna attitude from the beginning.