change one's tune - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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.