The audio has been confirmed to have been remixed. Probably because they wanted to do some sort of 7.1 HD audio version, so they decided to just start over. The picture I am sure will be pretty much the same; I can imagine them fixing some of the lightsaber issues, but I would say there is an equal chance that everything will be left exactly as they are.
As for whether the picture will be better than the broadcast versions enough to justify plunking down what will be probably $70-90...well, that all depends on how much you enjoy the SE. For some, there's no price that justifies it, for others they might be willing to if it was priced right. But as to whether the picture will be significantly improved period, I would say yes. The difference will not be extreme or anything, because some of the HD broadcast versions are quite good. But it definitely will look and sound much better on Blu Ray (well--in theory, anyway, you never know with LFL these days). And of course the set will have other value-added material, including some new docs but more importantly deleted scenes that I have a suspicion will be in HD. The deleted scenes will probably be put on Youtube (and yanked, and then put back on, etc.). In any case, if you really just wanted to see this stuff you can always rent the set; if I had a BD player that's what I would do, just to satisfy my curiosity of seeing the films in HD and also checking out the new supplementary stuff.
"I think when the Blu-rays come out owning the DVD's are no longer grey area enough for downloading HDTV rips. And so those will essentially become obsolete."
They will become obsolete, but only because the BD will be of higher quality so there would be no point in holding on to lesser transfers. Unless the films are changed again, but I personally doubt there will be any changes.