Oh Boy, Was I Wrong About Her

Tenkousaki no Seiso Karen na Bishoujo ga, Mukashi Danshi to Omotte Issho ni Asonda Osananajimi datta Ken · 転校先の清楚可憐な美少女が、昔男子と思って一緒に遊んだ幼馴染だった件

Seven years after making a childhood promise to always be friends, Hayato is reunited with his formerly tomboyish pal Haruki at a city high school–only now, Haruki has transformed into the girl every guy dreams about! While Haruki’s looks may have changed, with Hayato, she’s still the same pal he remembers. Despite years apart, their promise and feelings remain and deepen into something stronger. (Source: Crunchyroll)