I've seen the picture of this and would love to read the fan-fic if it is already out there.
Marinette is a twelve year old girl living in the country with her parents. One days she wakes up to find her self in the body of Adrien Agreste, a fourteen year old male model living in Paris. Adrien finds himself in a similar predicament when he wakes up in Marinette's body.
They find themselves switching bodies every so often, eventually trying to find each other.
Unless you've seen the movie, ages won't make sense. If you have, then the story would end where the Origins Episodes began. Teehee!
Whose Who:
Mitsuha - Marinette
Taki - Adrien