In Minders of Make-Believe Leonard Marcus says that Robert Cormier's first three books were for adults. His agent suggested submitting his fourth, The Chocolate War, as YA.
I've heard of that happening a number of times in recent years.
By the way, I have a Robert Cormier story:
I used to be interested in genealogy, before I finally accepted that it was way too exhausting and that there were far, far easier ways to waste time. Besides, one of my cousins found my family line back to the early 1800s posted on the Internet, so my work is done. But I was still dabbling in early Internet days, and in looking for French Canadian material, I stumbled upon a Franco-American site. At this site I learned that Robert Cormier was "our leading Franco-American writer." I found this interesting because I wasn't aware that "we" had a leading Franco-American writer, and how cool that if "we" did have one, he was YA.
Within a couple of months, I heard that Robert Cormier had died. God forgive me, my first thought was, "Does this mean the position of leading Franco-American writer is open? How do I apply?"
As it turns out, the competition for leading Franco-American writer is way too great for my taste.
I have, however, made a list of Famous Gauthiers.