As we all know so far, the members of the true organization are:
Xehanort
Ansem seeker of darkness.
Xemnas.
Young xehanort.
Marluxia.
Vanitas.
Larxene.
Xigbar.
Vexen.
Demyx.
Luxord.
Data Riku or repliku.
And Xion.
So, I believe that there is at least 1 person that will change sides, obviously the majority wants it to be Xion, but I believe that she is a 2.0 version of Xion created by vexen, with no emotions, so she is out of the picture, The person I believe that will change sides and help Sora is non other than Young xehanort, from the last trailer, we saw that he gives him advice on who to go after by telling him "find the hearts joined to yours" and warning him that his rash way of using his power will be his Downfall, further more in the opening cinematic we see a slightly younger version of him with dark gray eyes watching over a purple Myst that resembles Darkness, indicating that he had been possessed by it, I don't really know how he will change his course of actions, but my gut tells me that he will be the one to change sides.
If anyone has an other person in mind comment here and let's discuss ?