I think the rest of the park will be decorated by the time holiday small world opens...the "Believe in Holiday Magic" fireworks show is supposed to open the same weekend that holiday small world does. The Christmas parade is supposed to start on November 17, so I'm sure the park will be fully decorated by then!
I was there over Veteran's week 2 years ago and although the tree was up and they had the holiday fireworks etc the castle was not yet decorated so I think they keep gradually adding stuff and by Thanksgiving week or thereafter I think it is all up.