I am one who never believed Lucas retelling that he had never really planned nine movies. Back before Empire Strikes Back was released the number nine came up in news articles all the time, along with mention that the droids would be in every film.
My guess is that Lucas has suddenly remembered his original plan and turned the outline over to Disney.
Hamill is now the same age as Guinness during the original Star Wars, so it makes sense to do a movie set 40+ years after ROTJ with minor roles or cameos for the original cast.
Can't wait to see what comes out of this.