sing another tune

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 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.”