Lucas may have replaced Sebastian Shaw with Hayden Christiansen to increase the superficial similarity between the prequel trilogy and the Star Wars trilogy, but I doubt that was the only reason. Between 1999 and 2005, it was frequently reported that the nature of Force ghosts would be a major plot point to be resolved by the prequels. Ultimately, Lucas failed to provide a satisfying resolution to this plot, perhaps due to Liam Neeson's nonparticipation in RotS. (I have my own theories regarding Neeson and Qui-Gon Jinn's role in the prequels, but that's for another thread.) When he composed the list of substantive changes to be made to the Star Wars trilogy ca. 2004, Lucas still thought Force ghosts would be a major point in RotS. So, the change to a Force ghost in RotJ (DVD) was not haphazard; it was something he had been thinking about for some time, perhaps since he wrote The Phantom Menace. It is very probable that the insertion of Christiansen reflects a change in Lucas's conception of the Force. This change in the Force necessitates reconsidering the conflict between Luke and Obi-wan, and who was ultimately right.

Of course, there may be more at play here than Lucas simply changing his mind, or clarifying a previous point, about how the Force works. The new resolution to the Luke--Obi-wan conflict represents a radically different worldview than the resolution of 1983. It emphasizes the primacy of elders' wisdom over youthful qualities (naivete, hope, faith in daddy). This may or may not have occurred to Lucas when he made the change. The investigation is even more fun if we make the assumption that he was unaware of the full implications of his change.