No contracts have been signed but Hamill says Lucas won’t cast other actors in their parts if they weren’t interested and that he’d just write them out. Hamill said he’d prefer if the entire original cast returned.
Either way, I have no idea how Lucas will use the original cast. Hamill looks more like Darth Vader than Luke Skywalker now. It might turn into another Crystal Skull incident but instead of wondering “WTF, aliens?,” people will just be sitting there all depressed thinking to themselves, “Man, Luke really let himself go,” and “Slave Leia looks a lot different than I remember. Maybe they should just not save her this time.”