Fox has distribution on all the existing films until 2020. Then Disney gets distribution rights for 1-3, 5 & 6. Fox keeps distribution on 4 forever, or until Disney makes them one hell of a deal.
To further clarify: In the event the original versions ever see release, they won't be contingent on any sort of agreement from Fox. Fox has no say over the content of the films they're distributing, nor do they own rights to any specific cut or version of Star Wars. They simply own the rights to distribute the films theatrically and on home video until 2020, and for Star Wars (1977) in perpetuity.