Lucas says that Mark, Carrie, and Harrison were already on board before they sold to Disney.
http://www.superherohype.com/news/articles/175393-george-lucas-says-hamill-fisher-and-ford-are-pretty-much-signed-for-star-wars
Full story: http://www.businessweek.com/articles/2013-03-07/how-disney-bought-lucasfilm-and-its-plans-for-star-wars