Fox has video distribution rights. I think they could play it in theaters without Fox's consent. V and VI could apparently be distributed digitally without Fox?
But I think an important point that people sometimes gloss over is: would Fox try to withhold this? Why would Fox try to withhold this? Would they simply not want to put money into it themselves? I can see why Disney might not want to go through with it until Fox's rights expire (or are otherwise dealt with for IV), but I can't quite see what Fox has to lose, here.
Can we be certain about that? I recently read an article by Michael Kaminski who speculated that Fox had been offered the marketing rights for the prequels because Fox had something Lucas wanted in return and neither Warner or Universal could have give him: Star Wars - A New Hope
Therefore, it could be that Disney actually did acquire the rights for the entire Original Trilogy.
One of my friends who works with the industry told me many months ago, that the original version / theatrical edition would either be released to coincide with the theatrical premiere of TFA or its video release in 2016.
And the latest I heard about the basic story premise of TFA is that the Alliance wasn't all that victorious as the Special Edition of ROJ led us to believe, especially with its Coruscant revolution party (I always thought that the Coruscant team just won the superbowl and the party merely coincided with the defeat of the Emperor at the Battle of Endor - LOL).
Assuming that the SE of ROJ would constitute a continuity obstacle to TFA, then the reconstitution of the theatrical version (and its subsequent release) would already be one of the things we should be grateful for to TFA.