Ore no Tomodachi ♂♀ ga Kawaisugite Komaru!

Amano returns to his childhood city after a decade and reunites with Yukio Sawatari, a childhood friend, on the first day of his transfer. Yukio, who was a male friend, has suddenly transformed into a girl. Amano is happy for her but struggles with the situation, as Yukio is embarrassed by the sudden change. She is one of 100,000 people affected by a condition known as "spontaneous isomerism metamorphosis," which causes an abrupt transformation into the opposite gender.