They didn't have any problem recasting the role of Vito Corleone when circumstances made it necessary, why was recasting Clemenza (and later, in GF III, Tom Hagen) considered such a sacrilege?
I think part of the charm of the Godfather films is the continuation of the same actors playing the same parts. To divert from this might have weaken the films. Yes, Vito was recast, but since it was a long time line between, young Vito and older Vito, the casting was acceptable. With the mention of Clemenza's heart attack, it gave continuation of the story. I think the Pentangelli character fit right into the story line, and was well cast. There was a Philly mob figure that kinda looked like Pentangelli.
