nantaika

In Japanese, nantaika 男体化, meaning literally "male-body-fication," refers a woman to turning into a man, a girl into a boy, etc. Generally, this refers to gender bender scenarios in anime, in which a character changes sex through magical or sci-fi means.

For becoming female, the is nyotaika 女体化 instead.

Right: Uno Saki 卯野さき
Left: gender-bent version.
Anime: Mahou Shoujo Ore 魔法少女　俺 (Episode 1)