Keep everything from the 1977 version unaltered, but keep the "EPISODE IV - A NEW HOPE", keep the new Biggs scene, and you may fix the lightsabers, matte lines and lightsaber cuts. Keep the "close the blast doors" line too, and the scene which Chewbacca scares the little droid. The credits can be messed up as much as Lucasfilm wants to (credits James Earl Jones, copyright changes, THX).