Even though I do not like the change on the OT DVDs and would have been much happier without it, my explanation for it is simple; force "ghosts" only show themselves in the form they are most comfortable with. They are not a physical entity any more, they are pure spirits they have no real image, they left their physical bodies and now are pure energy, Obiwan and Yoda show themselves as their old selves because that's how Luke knew them and they are comfortable with that image, is just a visual representation of what they once were, just as with Anakin he is not young or old anymore he is a spirit, energy, one with the force, whatever you want to call it, and he is only showing himself as what he once was and obviously now that he has been brought back to the light he is more comfortable with his younger good image than with his old darkside self. It's not that he got rewarded for being bad or evil like some poeple have suggested, is his choice and I'm sure Obiwan and Yoda have a choice too, as i said that's only visual representation of who they once were and they do not look like that anymore, they have no physical bodies.