Also, maybe its just me and the type of personality I have, but even if i made changes I would keep a copy of the original, if only for myself. Im a programmer and if im radically changing the way i do some function or something I make a backup copy first (mainly in case i mess something up) and end up keeping this because its sometimes helpful to look back at how i did something previously.
And if the prints truely dont exists in the LucasFilm vaults I started a thread a while back that is probably lost at this point, after a discussion with a friend of mine who worked at the Library of Congress and the conclusion drawn was that they definatly have a copy of the original versions of the film where the only change that may be present is the inclusion of the Episode IV. Not that it would be possible to get their film verison of it to do a clean up restore on, but since they generally get 2 copies of everything (archive copy and public view copy), i would say the possiblity does exist (though i would figure it would have to be Lucas(Film) himself requesting it).
-Darth Simon