It was always planned that the original judges would rotate in and out, we all just assumed Mark Burnett was just saying that to let Christina save face.
Considering the Ratings have actually gone up, She must have good lawyers for this to happen. I'm guessing when she did the flopiatus they made sure she could go back whenever she wanted. Now Mark had to come up with away to keep the new more popular judges so it will be ceelo and christina in the fall seasons and shakira and Usher for spring seasons. I guess its not the worst thing for the viewers though, cause not sure I can handle Usher all the time.