I think it is better to replace Sebastian Shaw with Hayden Christensen completely in both scenes than to have them both playing Anakin in different scenes. The point is to have consistency. Luke knows only Shaw's Anakin. Someone who sees only ROTJ knows only of Shaw's Anakin. Shaw's scenes were not bad. If it aint broke, then don't fix it.

Same thing with Biggs in ANH. Include both Toshi Station and Biggs at Yavin, or none of those scenes. No more "Who the hell is that guy?"-moments, please!

Even though I prefer the O-OT, there are many improvements in the SE and 2004 editions that I don't mind, such as digital compositing, removed matte lines, flipped rank badges etc.
However, there are still many goofs left in the movie that I would not mind if they were fixed: Stormtroopers whose armor is falling apart, Vader's chest box being flipped in one shot.
Han Solo is also drawing a different blaster when he shoots Greedo than the one is wearing during the rest of the movie.
But if you are going to make a change, don't make it stand out!