Something that may be of interest in all of this is that the same starfield was used for the crawl on all three films. Of course, this excludes the original pre-Episode IV starfield, which is clearly different.

I'm with the general consensus that the starfield was changed when 'Episode IV' was added. I believe that when they created the new crawl, they used the starfield they had just done for The Empire Strikes Back, which was subsequently used again for Return of the Jedi. All of this makes the original crawl that much more unique.