A nice interview with writer, producer Tina Andrews.
She talks about The Brady Bunch around 21:37.
She mentions that she was over 18 and could get cast for looking younger than she was. And, indeed, she is around 5 years older than Maureen McCormick.
Davy Jones appeared on stage as himself in the "Getting Davy Jones" episode of The Real Live Brady Bunch on stage. It's good to see video from the performance posted on YouTube. This is not the original cast, but this cast includes Kate Flannery from "The Office"