I rather like Pterano's sleepwalking idea with all of them arriving at the same place, although I would find either possibility interesting. Pterano's idea would be quite different from what I used in my story, and could result in a much faster build-up to the gang being forced from the valley then what happened in my story (which took 3 chapters.)
With regards to dreaming I am open to either possibility but, like bushwacked said, it might feel a bit rushed if they sleepwalked and there was no intervening dream.
Also, how did rhombus handle the change in his story? I haven't had much time to read it, and I don't want to accidentally copy him
To summarize the events after the 'change' in my story:
1) Each of the gang was confronted with a graphic dream where they were chased (by their eventual species) and they sought refuge within a cave. They were eventually confronted with several paths, all of which were blocked except for one. After another chase (this time by the others members of the gang - now all sharpteeth) they are brought face to face with their own corpse. They then look into their own reflection in a nearby stream and see their new sharptooth form greeting them.
2) Upon waking, each one of them finds out about their change in different ways. For example, Littlefoot sees that he has sleepwalked some distance from his nest and decides to travel back, only to collapse onto the ground. Then he notices the change.
3) The gang then begins to join up and look for the others. Ducky and Spike were already in the same area so they form a group. Cera comes to the conclusion that the others must have changed too and begins her journey. Petrie eventually finds Littlefoot and those two also find the others. The reassembled gang of five then proceeds to journey to the Secret Caverns in order to find Chomper and Ruby.
4) Chomper is surprised to smell fast biter in the cave and hears Ruby whimpering. He panics and tracks the sound down to find Ruby (Now as a fast biter) crying. Thinking that the fast biter must have killed or injured Ruby, Chomper confronts her. Ruby then has the unenviable position of either proving her identity or becoming Chomper's first victim in the Great Valley...
Due to the different agent causing the change (Fate? in this RP as opposed to a ancient alien artifact that crashes into the Great Valley in my story) and the different options available to the gang (they initially try to undo the wish that they made to the artifact in my story, but this will not be an option in this RP) I don't think that there is much risk of repeating elements of my story in the RP. However, if anyone wants to borrow an element from my story and incorporate it into this RP then I am perfectly fine with that.

If you want to make sure that you don't copy me, however,
here is a link to the relevant chapter in my story. The events described above continue in chapters 7 and 8.