The thing is, I don't think GL's an artist. He's a conceptualist who thinks he's an artist. The man has fantastic ideas, can establish great foundations, and has vision. What he's lacking is talent in storytelling, continuity, characterization, themes, consistency, and directing. These are all necessary elements of anyone hoping to be an artist in movie-making and he's sorely lacking in these areas.
Does GL have the right to rerelease these films? Absolutely. The problem is I don't believe that he's really thought about his films and how, from a storytelling perspective, some of the changes are damaging to the whole that he and others (let's not forget that the others are probably reason why TESB is the best of the series to date) have alread completed. Jabba the Hutt and the ice monster of Hoth are two examples of this. Jabba was effective because he hear about him in two movies and can appreciate just how dangerous he is... So much so that he's able to reach into the Rebel Alliance and almost assassinate/capture Solo between ANH and TESB. But never seeing him is important because it builds up his intensity as a great villain that we can't wait to eventually see. Meeting the Jabba in ANH, who is clearly not nearly as sinister as the Jabba in RotJ (because the writers were probably much different by the time we moved from IV to VI), is a let down in anticipation and doesn't convey as powerful a story. The same thing with the ice monster. It was more effective in the original showing because we only capture glimpses of it. The added scenes in TESB do nothing to carry the story forward (this is especially the case of the extended scenes with Vader returning to the Executor towards the end of TESB, which did nothing more than chop up the anxiety and tension of the escape from Cloud City).
Characterization is another area in which Lucas has little talent. His handling of the characters in the prequels clearly reveals this. It's taken strong actors to inject personality into the characters that he "directs," which is why only Kenobi and, at times, Amidala, seem worth watching. The whole "Solo shoots first" is an example of this (although I've heard that some fans are also to blame). That single scene solidifies who and what Solo is: he's a self-centered, out for himself, pirate with a soft spot.
Now, there are plenty of changes that made sense. The entire dogfight at the end of ANH made sense. The revamped scenery for Mos Isely made sense. The revamped scenery for Cloud City made sense. These changes propelled the story and intensified our suspension of disbelief, placing us more firmly in this new galaxy far away. Adding Ian McDiarmid to TESB makes sense for continuity purposes. Hell, redoing the score for ANH to include "The Imperial March" would make sense too for the same reason.
The problem with Lucas is that he doesn't know how to discern the need for a change. He doesn't like it, he changes it without really thinking about how the change affects the overall quality of the film. I agree with him... He shouldn't worry about a bunch of rabid fans who cling hopelessly to some element of their childhoods, unable to understand the need to pursue a vision that tells and shows a better story. Unfortunately, Lucas himself has no idea how to tell a better story because that's not where his talents lie. So, we end up with a mess that neither pleases fans nor makes sense to those more circumspect in nature. It pleases Lucas who changes because he thinks it makes sense when, in the end, he's ending up with an uneven end result.