I suppose you could say that time travel is similar to having a dream. You leave your body, and your heart/mind move through time. You can only move one way, and can only go to times in which some version of you is active. You cannot change what has been destined to occur. Also, apparently there is a certain amount of time you can spend outside of your body. When this alotted time is over, your heart returns to your body at the exact moment it left. The memories are forgotten. But, the future engraves that destiny in your heart, leading you to become that future. This feeling of destiny can be associated with deja vu.
YX even mentioned deja vu to Sora in his sleep. He was referring to destiny, and how it feels like it.
If you pay attention to these rules, it seems anyone with a heart can travel through time. All you must do is concentrate, and leave your body behind. For beings of just the Heart, it is easier, for the body has already been discarded. It is just the Heart/Mind that moves through time. This means all the members gathered through time are floating hearts.
However, when you do travel, you create a Time-body, a PPS made to give the heart a physical form. This Time-body takes on the shape of the body you discarded to travel through time.
All the members gathered in WNG dissappear at the end, including the ones who should exist in the present. It appears that they all are returning to their times. For those in the present, this could be just a day or so before. What this means, is that the only real bodies in the room are Sora, Riku, and Mickey (until Lea, Donald, and Goofy arrive).
An interesting observation: MX dissappears along with everyone else in the room, meaning he traveled back to his time as well. Him "forming" was merely him arriving at that place in time.
The time he returns to is when he reformed (for real) in Radiant Garden, along with Braig, Isa, and some form of Terra. All of these return to this moment, and leave before Lea, Aeleus, and Ienzo come to. They were all able to travel to WNG through time, including Braig and Isa. Meaning, anyone can travel through time. They just have to have their heart leave their body at some point in their life.
What this means is, Sora, Riku and Kairi can probably do this as well. All they need to do is learn how, like ASoD told YX how.
Time travel won't help the Lights prevent what has happened, however, because they cannot change what is destined. They should, though, also be able to gather people through time. Like Roxas, Xion, Naminé, etc.
So, what do you think?