The problem is that Shaw looks nothing like Christensen (and Lloyd looks like neither though children do change a lot). McGregor and Guinness actually look alike (they go into it in the special features) which makes the transition much easier (even if the ageing is weird). Like I said, it’s not a massive problem in the unmasking scene (once the features continuity is fixed), but as a ghost, he is completely unrecognisable as Anakin in face, style, posture, or voice.
Though, as Hal has indicated he is using the Christensen version in RoS:A and I’d got the wrong end of the stick, I’m just contributing to the gargantuan mass of this thread.
As I thought about the possibility we could see Anakin in TROS before it’s release, I hoped they would at least let Hayden appear somewhat older than he did in ROTS. No long hair, but not necessarily elderly either. Just to break from “I am literally exactly how I was right before turning evil.” It probably would’ve looked too silly, but I’d have even liked to have him made up to resemble Shaw, maybe just his hair even.
It’d have to be all done digitally because neither Christensen or McGregor have aged significantly since 2005. So just sticking his face into an ager probably wouldn’t do much 😄 .