I'm pretty sure they will allow some minor flexibility...but not a whole lot.
I think they will do what they can to coordinate with your colors (when you decide on them

) and, of course, the flavors. But I think you are kinda stuck a bit with the shape and frosting style.
I have heard of some brides requesting fondant instead of the buttercream frosting and they are charged a nominal fee (I want to say $75, but I'm not entirely sure).
I know the white chocolate castle topper is extra as well.
What kinds of changes are you thinking of?