"Star Wars," the original film released in 1977, was a ground-breaking effort, although Geoge Lucas subsequently said a lot of the special effects were limited by the rudimentary technology of the time. And he declared himself dissatisfied.
When the third movie in the trilogy, "Return of the Jedi," was released, Lucas hinted he had some ideas for a second trilogy that would take place years before the first. He knew if he made sequels, he would have to wait years for the original actors to grow old. (Now might be a good time.) Prequels made sense if he was to extend the world of the Jedi knights.
Anyway, when the first "prequel" was about to hit theaters, Lucasfilm announced that, henceforth, the first "Star Wars" movie, always simply known as "Star Wars," would be retitled Part 4, and given a new name: "Star Wars: The New Hope." So the first three films then became parts 4-6. The prequels suddenly became parts 1, 2 and 3. Never mind that the production design, if not the actual stories, of the prequels far outshone the originals. The newer films could offer effects that could only be dreamed about in the 70's.
When Lucas saw what he could now do with the more advanced CGI, he started tinkering with the original three movies, adding whole sequences that he thought would freshen the franchise. Purists were aghast. Some fans didn't care. And, hey, it seemed to sell more DVD's, so Lucas never looked back. His goal was that all six movies would have similar production values and a seamless look.
It's troubling and sad that Lucas won't release the original movies in Blu-ray in their unadulterated form. I have read elsewhere that it's because his ex-wife owns partial rights to the originals and he can't release them without making her quite rich. So, to spite her--and ultimately, fans--for the near term, we will not be able to view them the way they should be seen.