My children are currently watching the Berenstain Bears, on PBS, based on the books by Stan and Jan Berenstain. I remember reading a book called "The Spooky Old Tree," when I was the Boy's age. It was a book written by Stan and Jan Berenstein - a Berenstein Bears book.
I don't know when the change came, and I'm not sure (though I have a suspicion as to) why they changed. I think it's unfortunate that whoever made the decision to change the name felt obligated to.