change (one's) stripes

To radically or fundamentally shift one's opinion, ideology, or allegiance, especially in relation to politics. Often used in negative constructions. Derived from the phrase "a tiger cannot change its stripes," meaning that people are incapable of changing their essential nature. The former governor has changed her stripes regarding social welfare since leaving office. You're never going to convince Tom about this. A guy so set in his beliefs like Tom can't change his stripes.

see under leopard cannot change its spots.

One cannot alter one’s essential nature. The cliché refers to the tiger, who obviously cannot change its stripes. This in turn is an alteration of a much older saying, an ancient Greek proverb also found in the Bible (“Can the Ethiopian change his skin, or the leopard his spots?” Jeremiah 13:23). The phrase appears in numerous places until the end of the seventeenth century.