Tensei Shitara Warui Kuni no Musume DeshitaTensei Shitara Waruiko o Deshita; 転生したら悪い国の娘でした。; 轉生之后變成壞女孩; I (♂) was Reincarnated as a Female Member of the Enemy Country

Childhood friend (♀) gets attacked by something (♀), Main character (♂) dies in crossfire. Main character (♂) is reincarnated as the living weapon (♀) of the demon lord (♂->♀) that just killed him, while his childhood friend (♀) is apparently now the hero (♀->♂?). Reincarnation + Isekai + Double(?) Genderswap.