Godfather and the Pacino/De Niro switcheroo

I have flipped by The Godfather Part II a couple of times in the last month or so, and it has helped me realized why I am always confusing Robert De Niro and Al Pacino. It's because somewhere along the way, possibly starting during this movie, they started to switch bodies. To wit:

Pacino (now): Frizzy-haired, craggy, gaunt, looks exhausted.
De Niro (now): Kind of pudgy face, looks amused.

But! In the Godfather II days, it was all bass-ackwards:

Pacino (then): Smooth-haired, smooth-faced, looks like he used to have inner peace but now it's under siege.
De Niro (then): Good cheekbones, attractive, looks like a moral man forced by circumstance into a life of crime.

If you were going to pick which young guy would grow up into which old guy, you would totally switch them around. See, I'm not just ignorant about classic actors (or Italian-Americans)--it's just that my understanding of them is deeper than everybody else's.