The position of singer is probably the hardest to fill for a band that's already successful. That person's voice is a part of what made your band successful to begin with, after all. You can go the Journey route and recruit a guy who's a replica of your previous guy or you can take your chances on someone with their own unique voice.
I'll start it off with Bon Scott to Brian Johnson. That's a hella transition. Both brilliant singers with their own distinctive styles but both worked perfectly with AC/DC's sound. Win.