Gandalf does seem alot different between the Hobbit and LOTR. It might be of a specific audience but, i don't feel Tolkien was trying to convey that message... More or less, the Hobbit seems to be the "test drive" of middle earth and what it holds. You seem to be explained to by what characters learn in the Hobbit but, in the LOTR, things seem to be expected. As with gandalf, i really only see character development, or, how the main character conveys his personal image of Gandalf. Bilbo seems to view Gandalf much differently than that of Frodo who, in the beginning of the lord of the rings, bases most of his knowledge of gandalf by Bilbo's tales and gandalf's actions around him and other hobbits. As the book progresses, we see Gandalf literally, take on a new light. Frodo notices this as well, and begins to see a 'new' gandalf that he hasn't experienced before. Thus, we see through a new image.
Another factor can be that in the Hobbit, 90% of the point of view comes from Bilbo, while in the LOTR, more character's share their thoughts and views in the story.
Another person who changes, or we have a chance to see and hear more of is Elrond. Who, in the LOTR, plays more in the plot of the story than in the Hobbit. Explainning a map vs. Holding a council, frustration with in-laws and other activities...
I see a change...
~Ka