change one's tune, to

To change one's opinion, behavior, or attitude, especially suddenly or abruptly. He never used to support that political candidate, but he's singing another tune all of a sudden. I used to be very cynical about the world, but ever since surviving that car wreck, I've been singing another tune! They'll be singing another tune after they see what we've come up with.

to change the manner of a person, usually from bad to good, or from rude to pleasant. The teller was most unpleasant until she learned that I'm a bank director. Then she changed her tune. "I will help change your tune by fining you $150," said the judge to the rude defendant.

Also, sing another or a different tune . Reverse one's views or behavior, switch sides, as in When she realized she was talking to the bank president, she quickly changed her tune, or I bet Dan will sing a different tune when he finds out what the salary is. A version of this term, sing another song, dates from about 1300, and it has been theorized that it alludes to itinerant minstrels who changed the words of their songs to please their current audience. The first locution was already in use by 1600. Also see dance to another tune.

To alter one's approach or attitude.

To reverse one’s views, change one’s mind, switch sides in a controversy. The analogy is very old; John Gower wrote, ca. 1394, “Now schalt thou singe an other song,” and the actual phrase, “change your tune,” appears in a ballad about Robin Hood (one of the Child ballads) from about 1600. And a character in Samuel Beckett’s novel, The Unnameable (1953), says, “I have my faults, but changing my tune is not one of them.”