Making a mob change form

I wanna make a mob that looks like a passive mob but turns into a hostile mob when it notices the player. I was thinking the best way to do this would be to have the original mob die when it sees the player and have the hostile mob spawn in it's place. Is there a way to make a mob react this way when it notices the player? And how do I make it so it'll only do it if the player is seen by the mob or if the player gets within a certain distance of the mob?