Kashimashi: Girl Meets Girl - Shoujo wa Shoujo ni Koi wo Shita

This is an OVA released on October 27th, 2006. It is also classified as the 13th episode (thus a sequel) to Kashimashi ~Girl Meets Girl~. Hazumu confessed his love to Yasuna, but she turned him down him. To ease his heartbreak, he went to Mt. Kashimayama where he had met her first. However he lost his way in the mountain, and it was night time. He saw a big shooting star, and when he began to wish, he found out that something was wrong. It was not a shooting star, but it was a falling space ship. He was involved in the crash, but he managed to survive by the help of an alien. However, he became a girl due to the accident during the reconstruction of his body. (Source: ANN)

Plot summary from MyAnimeList. Data and images via Jikan API.