chang one's tune : 태도/의견/입장을 싹 바꾸다.
to chang your opinion completely, especially beacuse you know it will bring you an advantage.
[Desperate Housewives Season 6/ Episode 02]
[Gabby] : Ana was lying.
[Danny's mom] : Well, why would she lie?
[Gabby] : Because your son told her to.
[Danny's mom] : maybe she was changing her tune because she didn't want to get in trouble with you.
[Gabby] : No, no. She's covering for Danny.
[Danny's mom] : I don't know what you're implying, but he's a good kid.
Your niece is the one who's been hounding him day and night.
[Gabby] : Hey, I'm just trying to be nice here! I'm giving you a heads up that Ana's taking her story back.