Itsuka Sora kara

Nakagawa Yuki, a popular boy at school, lives a fulfilling life, always assuring his friends Shinichi and Midori that he is happy. However, on his 17th birthday, his father tells him, "You're an alien," and he discovers that he has awakened to the ability to transform into other people. Then, by chance, Shinichi and Midori see Yuki transformed into a girl, and Shinichi falls in love with him at first sight...

(Source: Natalie, translated)