The writers and Abrams would have have some sort of on-screen explanation for why Obi-Wan's ghost spent years appearing to Luke as Alec Guinness and then all of a sudden is a generation younger and played by a different actor, but it's the Force and the supernatural. All Obi-Wan has to tell Luke is "this is how I appeared when I was a much younger man in the prime of my life....when your father and I were good friends."
Then for fun they can use CGI to have Ewan morph into Alec and back. It's a modern Star Wars
movie. Wasting CGI on something minor is gonna happen whether Lucas is in charge or not.