I've seen a couple of singer switches happen right before my eyes, one being Roy Khan to Tommy Karevik in Kamelot, and the other was Anette Olzon to Floor Jansen in Nightwish. Those happened with me present, and I saw how well the fans took to these singers because they were presented in the proper way.
Forgetting Sarah Marshall
Rachel Jansen: [on the phone] Peter
Peter Bretter: Sarah?
Rachel Jansen: No, Rachel Jansen from the front desk whats going on up there, we're getting calls about a woman crying hysterically
Peter Bretter: Yeah, y'know,I hear it too, it sounds like she's having a tough time, I think its coming from the floor
above me
Rachel Jansen: You're on the top floor.
Peter Bretter: I'll try to keep it down.