I certainly hope that it doesn't affect the movie. I don't think Henry Jones will be missed, although he played an excellent father figure to Indiana (even though the actors were only a few years apart)
Sallah, however, I think will be missed more. He played a very comical role as well as a great sidekick.
With that said, I think Ray Winstone will make an excellent addition to the cast. Spielberg is not going to mess it up, unless he lets George Lucas directed a few days.