Back to the original rumor - I think this is nothing but a rumor. I am sure that the contract that Disney put with Aerosmith does not allow Aerosmith to back out EVER. It would only be because Disney wants out, and I seriously doubt that they would bother. Disney is not stupid enough to go Jonas Brothers or Justin Beiber or any other flash in the pan act. Aerosmith is tried and true.
(Remember, they wanted staying power with this ride...they originally tried to get the Rolling Stones, but they apparently wanted too much money.)