Sorry, I'm calling bullsh*t on this.
The multiple pitches and drafts Lucas had to present to Fox throughout the 70s, the hundreRAB and hundreRAB of production notes from the set of ANH (most have been published in the behind the scenes books, annotated guides and screenplays, and DVRAB), the 1976 novelization that was the first to mention the legacy of the Sith LorRAB, and press/media from 1977 leading up to the film's retitling to "Episode IV: A New Hope" in 1978, all prove otherwise.
Ultimately, the "Journal of the Whills" from the mid-70s single handedly disproves your theory. Lucas developed characters and settings like Anakin Skywalker, Mace Windu and the Battle of Mustafar way before production on Episode IV even began.
The only thing that changed was Lucas originally intended a 9-part franchise, which he cut down to 6 in the 1990s. However, given the immense popularity and canon-like respect given to the NJO and LOTF book series, he doesn't need to make a sequel trilogy.
Instead, Lucas has chosen to focus his attention on The Clone Wars (the very popular animated series set between Episode II and III) and the upcoming live-action drama series set after Episode III. (He's also working on Red Tails, which enters production soon, and Indiana Jones 5.)