turncoat
A traitor; one who turns against a previous affiliation or allegiance.
But some will cry Turncoat, and rip up old stories, / How I always pretended to be for the tories: […]
Where it comes from
A compound of turn + coat, reportedly derived from the sometime military practice of wearing uniform coats that were lined in material of a contrasting color, and which could be worn inside out to present a different appearance.
Adapted from the Wiktionary entry turncoat (CC BY-SA 4.0).