The original cast are getting pretty old. I'd find it very hard to get past that everytime one of them was on screen.
Harris Ford I could just about accept but not Mark Hamill. Maybe if Luke was dead and only in the movie as a Jedi ghost I could accept this. They could use Hamill for the voice and movement capture but the actual ghost would be a CGI version of Skywalker from around Return of The Jedi.
In fact if his family don't object I'd be happy with a brief ghostly visit from Alec Guiness too!