Wow. That's um...
Okay, no. That's not going to happen. Why the f*** would they hire a different guy to play the same character? Don't you think that maybe their decision to hire the guy was a little more informed than that? Do you honestly think they're going into this having no clue if he's going to be good for the part or not? They hired him because they're confident he can do the role. They didn't pick a name out of a hat, and they're not going to hire a different actor to play the Leader. I mean, why would that happen?
"Okay, well, we've done a good job with the casting so far, but this Leader thing is really breaking our balls... there isn't a single person who seems capable of playing this character pre and post transformation! We have no choice but to hire two different actors!"
Is it possible he won't perform well in the role? Sure. But I can guarantee, they hired him because they think he WILL. They didn't just forget that the actor has to be able to play menial chemical plant worker Samuel Sterns AND super-smart supervillain The Leader. And the fact that you have trouble seeing him in the role, or that you're too attached to the cartoon voice actor isn't going to make a difference.