I've heard about this, and I am not sure whether its true or not. I would love it if it was true. McKellen is a superb actor, and has clearly shown he can bring a literary wizard to the screen wonderfully(Gandalf). Although, IMDB and The Sun(where I heard of this) are not reliable sources. The Sun was the one who reported that Dumbledore's face would be digitally added on to a double. I'll wait until David Heyman makes this official.
P.S. I am going to merge this with the PoA countdown.