The films are already out on DVD, at least unofficially in Fan preservations. Thats why most of us are here. Hopefully someday we will see official releases, but I don't expect any until Lucas retires, has a massive change in heart, or hits bottom and has to release the originals because he needs the cash.

As for why does Empire have the least changes, I think it is because it 'needed' the least changes. It didn't have any musical numbers, earlier film techology, etc, and no contridictary references to the preqels (Anakin force ghost). Although Empire is seeing its fair number of changes, such as the poorly done Ian McDiarmid as the emperor instead of Clive Revel.