Atlas Shrugged Part I and II

Just watched both of these for the first time today. The first one brought me in and I wanted to see the conclusion. A minute into Part II I realized that a main actor was replaced. A couple of minutes later I realized all the main actors were replaced with new actors. I know that movies and TV shows replace an actor here and there because of scheduling and contrat issues, but is there any other production that has replaced all the main actors with new ones? I mean these two movies were shot about one year apart and all new. While I enjoyed Part II it was hard to relate to the new actors having seen Part I earlier in the day. What are your thoughts on actor replacement?