But you cannot re-cast actors who were in the movies for different roles.

Vito - Brian Cox
Michael - Vince Vaughn
Sonny - Colin Ferrel or Joaquin Pheonix
Fredo - John Favreau
Tom - Matt Damon
Connie - Selma Blair
Kay - Reese Witherspoon
Appalonia - Jamie Lynn Seigler
Clemenza - Danny DeVito
Tessio - Eugene Levy (sorry - the eye brows are hard to resist)
Luca Brasi - The Rock
Barzini - Christopher Walken
Enzo the baker - Chris Owen
McClusky - Gary Oldman
The Turk - Michael Madsen