I think there is something of a tradition in fiction that you do not mess with the first published edition of a work willy nilly.
Not really. Plenty of rewrites and "retcons" in fiction throughout the ages.
Retconning, while commonplace, is usually employed in order to make subsequent works make sense, rather than because you feel like changing it.
Incorrect assumption. Authors (or other creators) change their prior works for all sorts of reasons.
In this sense, Greedo shooting first had absolutely nothing to do with the plot of the prequels. Even editing Hayden Christiansen into the celebration scene is far more justifiable (though hardly necessary, and definitely in poor taste), because you could at least say that that is what he looked like before his fall to the Dark Side.
All the changes to Star Wars that you listed, I don't personally care for. As I suspect, do most folks posting in this thread, and many other fans. However, I think it's Lucas full right to make whatever changes he wants for whatever reasons he wants, whether I end up liking them or not. And when I don't care for the changes to a prior work, I don't get so worked up about it that I cry my childhood was stolen from me, or other similar histrionics (not that you do Kaodi, just a general comment).
I think if we want to really figure this sort of situation out, we should ask ourselves why good ol' J. R. R. Tolkien does not get (much, if any) flak for changing the chapter with Gollum in The Hobbit. As I mentioned above, Tolkien's retcon of this chapter was to set up for The Lord of the Rings. But then, Wikipedia tells me something else I had not known: that Tolkien actually started a rewrite of The Hobbit in order to bring the tone more in line with The Lord of the Rings, but ended up abandoning it because he felt it just was not The Hobbit.
Tolkien is not alone in making changes to prior, beloved works, or in starting to make some changes and backing off. Again, totally his right as the creator, IMO. One reason Tolkien doesn't get as much flak as Lucas (although I bet there are some serious nerd fights on Tolkien fan boards on this issue), is that his changes seem to work pretty well. Also, they were made ages ago before the internet, before it became easy to notice and compare changes to prior works, and before the rise of the angry super nerd culture.
( Another question I thought of, though it is probably somewhat less applicable: if Leonardo da Vinci were to be resurrected from the dead, would you let him paint over the Mona Lisa with a new version of the portrait, or would you tell him to take a hike? )
Poor analogy, because Lucas didn't destroy the originals to create his revised "masterpieces". He has certainly tried to suppress the original films in favor of the reworked ones from time to time, but they are still around to enjoy.
If Da Vinci came back to life, purchased the original Mona Lisa from the Louvre, and THEN proceeded to paint over it with a "new-and-improved" version . . . . well, that would be a damn shame. But again, well within his rights to do so. Heck, if I purchased the Mona Lisa and did that, it'd be within my rights to do it . . . .
Art is powerful, important, and meaningful to society and to individuals, as it should be. But at the end of the day, it's just a movie, just a book, or just a painting. We can always make more.