Spoilers in white text :
I understand some spoilers have been released on a HP site concerning a few of the chapters, one of them supposedly called 'Draco's Detour' which set off all sorts of speculation. I have no doubt Draco will remain the nasty bully JKR intends him to be, so I have to wonder if this isnt another one of her famous red herrings. Personally, I think if she doesnt start wrapping up loose ends in this 6th books, Im going to be a bit put out. No one should have characters in the 3rd, 4th, 5th, or even 6th act of a story that are the same as they were in the 1st and 2nd. That includes Snape, Dumbledore, and others as well as Draco.
Heck, in LOTR we knew by the end of Two Towers that the hobbits had grown beyond who they were in Fellowship. Why do we have to wait until the 7th act to have it all become so clear?
Rant /OFF.
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.