I wouldn't be surprised if, at some point, the DC queue were rerouted away from the Hogwarts stuff, and that part of the area were devoted to the queue of the much-rumored future new HP attraction, to be based on the 7th and 8th films. But no sources have confirmed that to me.
Also, to clarify, the queue for Forbidden Journey will take you through the entrance hall and the Great Hall of Hogwarts.
I don't see how you'll be able to go through Hogwarts to get to Dragon Challenge. The two are separated by Flight of the Hippogriff.
Matt
Orlando Attractions Magazine