From past threads I remember there was a problem with Richard Castellano, the actor who played Clemenza. He wanted more out of GFII than FFC was prepared to give. So the older Clemenza was written out (that was no heart attack) paving the way for Frankie.

And yes, I'm sure there may have been somewhat of a different story had Clemenza been able to stay in the picture. But remember, a main theme of GFII was Michael's growing distrust and hardness due to betrayal coming from just about every direction. So it would've been interesting to see if even Clemenza had remained loyal.

Apple