When Castellano finally said no, Coppolla wrote in his letter:
"I wrote the part for you but now I'm forced to change his name to Willie Cicci. ... and just anoter actor play it. He will arrive in the first scene, be introduced as an old time affiliateof Don Corleone's in Brooklyn... and the role will wiork exactly as it did for Clemenza. Out of respect for you, I will not call him Clemenza. (...)"
This tells me, that the basic storyline hasn't changed so much, especially the betrayal. However, they changed Cicci, and called the guy Pentangeli.