This is similar to a body switch story but not exactly.

Firefly lives a almost normal life in Austin, Texas. Plenty of friends, creative, caring, kind, and crazy. Or as everybody calls her... Weird. She is happy. Her life is finally worth living and she's glad. She has love, drama, tears, laughter, everything she ever wanted out of it. Well not completely. She wants to be special. She wants to be special with powers and maybe to even have a sister.

Rainbow Dash lives in Vancover, Canada. She has her friends, her life is just fine. Its only fine though. She doesnt have a family so she lives now with her best friend. Next year she will live in a high school dorm. This isnt the life she wanted. That is until she sees herself on TV winning a best authors award. She comes up with a plan. This plan might change the course of the future and everybody in both Firefly and Rainbows lives.

Picture is of ponies but its humanized! Best I could find for the book.