The release is taking place two weeks earlier, probably to avoid competition from the DreamWorks film, Transformers, which is being released over the Independence Day holiday in the United States.
The same thing occured when the first Fantastic Four film was released. At that time, the film was released earlier to avoid competition from Steven Spielberg’s War of the Worlds, which starred Tom Cruise.
Fantastic Four 2 will be taking the place of the scheduled release of the comedy Used Guys from director Jay Roach (Meet the Parents).