I'm not so sure. FFC knew so many tricks.
Let's say, Brando had decided to play young Vito. And now we'd be here discussing what would have happened if De Niro had played the role. I guess, that most of us would consider just the idea of it distasteful.
Or if FFC had decided that Fredo would not be killed.

The same here: We don't know how FFC would have written the story with Clemenza. One thing is sure: He knew how to surprise us.
I believe, the party scene ould have been different. Clemenza would not be as drunk as Pentangeli. He would try to talk reasonably to Michael. Perhaps there would have been some parallels between young and old Clemenza. Well, we never know :-(