The voice is a problem. The voice actor cannot change just because of what we learned from the manga. I guess its possible that it will change once he is unmasked, but until then, it certainly won't change.
The bottom line, as others have said, is that Tobi was voice cast as Madara Uchiha. So it is going to be a problem for them to get a new voice actor. I guess its possible to cast the same actor and have him change up the voice a little from Tobi's, but clearly since Tobi was originally voice cast as Madara causes a dilemma in itself. I think we have all considered this at least once, its clearly a problem that Kishi had to let happen to keep his secret on the matter.