http://img291.imageshack.us/full.php?image=ddd1hw.png
1. Take in the color from all their hair. Kairi's is a slightly dark red, Selphie's is light brown, while BHK's friend is dark brown. The girl in the trailer seems to be dark brown.
2. The length and styles of hair. If you see Kairi's hair length is goes way past the girl in the trailers hair, past her neck. Selphie obviously hasn't changed much since KH1, and i see no curls. And BHK's friend has these little spike thingys on the side of her hair that are noticable.
http://img291.imageshack.us/full.php?image=ggg9sh.png
3. Kairi's and BHK's height are slightly similar. BHK's isn't that short when compared to axel's, so Kairi shouldn't appear so small as well.
4. I didn't even bother to include Kairi's clothes since they are obviously different. Selphie's much more yellow, and doesn't have a belt around her waist. BHK's friend has the color down but missing the skirt.
5. If Kairi all of a sudden changed clothes, why has all the official artwork have her in the clothes we've seen her in through the trailer.
So i believe it is neither Kairi, Selphie, or bHK's friend but a possible new character.