He probably already knew there was a sketch out there. I think he would have looked up the Fairfax news in the months after the 2005 attack, and therefore knew the victim was alive and had made a sketch. He maybe started changing his appearance in subtle ways after killing Morgan just incase the cases should be linked. Then once they were linked and the sketch was circulated in Charlottesville, and people commented on it looking like him, maybe he panicked and couldn't change his appearance subtly anymore, he had to go all out.
Or maybe I am giving this "slow" guy too much credit. But then, he has evaded being caught for a very long time. I don't think he is slow at all.