Cinderella Boy (2003)

Ranma and his partner Rella run a detective agency in Kirin Town. An arranged accident leaves Rellaa and Ranma in an unfortunate state where the two detectives swap bodies at the stroke of midnight. With Ranma dissapearing and the beautiful Rella appearing and viceversa. The two must discover who's the responsible for this mysterious experiment on their bodies and for what cause, meanwhile running their Detective Agency, the R&R.