Sakurai Mei no Tsukurikata

Ryuuko and Sakurai Mei are childhood friends. After separated for several years, Ryuuko returns with the intention to confess his love for her. Out of his surprise, Sakurai Mei is so different now—from a cheerful and shining girl she turned into to a gloomy loner. Fortunately(?), a miracle happens and their bodies are swapped. Now it's Ryuuko's chance to return Mei's smile by becoming the old her!

(Source: MU)