No, some of the characters you mention change, but she sets up the change at the very beginning. Look at Neville. Sure, he majorly improves in OotP, but that was hinted at in the first book.
And Snape is of course a horse of a totally different color.
I still feel kinda sorry for Snape...almost like the guy.
Of course, part of that may just be Alan Rickman's fault, but...