A reason for Sidious' appearance change...
I don't think I've seen this anywhere, but I wonder if Sidious' appearance changed because he had to use the force to stop Anakin from dying after he fell into the lava and it drained him so much? If Sidious is there, surely he'd attack Obi after Anakin falls, unless he's preoccupied? Yeah, his appearance could change purely because of his allegiance but maybe this has some part to play?