change one's tune

idiom

informal

: to change the way one talks about something : to have a different opinion about something

He bragged that the test was easy, but when he saw his grade he changed his tune.

… one weather center based in Europe predicted the path to swing up, but no one was taking it seriously. There was nothing to worry about. By the following afternoon, everyone had changed their tune. This storm was coming.

—Jaron Schneider