A volte-face is a sudden reversal of an opinion or policy. A lifelong fan of the New York Yankees who abruptly starts rooting for the team's archrival, the Boston Red Sox, has made a volte-face.
The French volte-face literally means "turn face," and its English synonym is about-face. A surprising reversal of political beliefs, a sudden passion for watching television after years of claiming it rots your brain, or a government changing its allegiance from one alliance of countries to another — these are examples of making a volte-face.