Turn-coat

You say you have no use for turn-coats, what use have you then for Luther, Calvin, Zwingli and Henry VIII? all turn-coats, because all were born Catholics.

Source: wiktionary

On a visit to some friends at Worcester, he had the piece with him; meaning I suppose, to afford them a little amusement, at Southey's expense, he being held in great reproach, even contempt, as a turn-coat.

Source: wiktionary

Indeed, he gave a lucid self-diagnosis of these competing tendencies when referring to the irony of his defection from Britain at the beginning of the First World War, describing himself as 'a coward, a turn-coat, whose lifework was to celebrate in music beauty-born-of-bravery!'

Source: wiktionary