I'm currently reading Rinzler's books about the Making of Star Wars - New Hope was originally going to be Episode Six; One and Five were going to be a prelude and epilogue respectively, and Two to Four were going to be the Clone Wars trilogy.
Doesn't mean the entire thing was deeply planned out, or that each episode was written or mapped out, or anything else. Just that the Clone Wars were going to be more of a thing than they were.
(I've not got very far into the Empire book yet, so I don't know if the plan changes were explained)