You've changed your tune

Li: Hello and welcome to The English We Speak. My name is Yang Li.
William: (mouth full) And I'm William, mmf, mmf.
Li: My dear co-presenter William! Don't you know it's rude to speak with your mouth full? Particularly in a BBC audio programme!
William: (smacking lips) Well, I do know that but I've really been enjoying this super jumbo hamburger. Wow, that was so good. I just can't get enough of super jumbo hamburgers recently.
Li: Oh? So what is it about super jumbo hamburgers that you like so much?
William: Well, good question Li. I think it might be the delicious 100% beef burger and how the flavours merge with the sauce, which is made to a carefully-guarded recipe. But then again I like the lovely gherkins and the crisp crunchy lettuce. But also let's not forget the layers of soft yellow cheese. Maybe it's all of those things. Hmmm. I think I could have another actually.
Li: Well Will, all I can say is: you have changed your tune.
William: Hmm ... That's an interesting phrase, Li. Shall we make it our phrase of the day?
Li: Sure, why not?

If you change your tune, you change your attitude towards something or someone.

William: It's a slightly critical phrase, but in a jokey sort of way. Let's hear another example:
Girl 1:What do you think about Mark? Don't you think he's really good-looking?
Girl 2:Mark? I thought you hated him?
Girl 1:That was last week. This week, I feel very different ... 
Girl 2:Wow, you've certainly changed your tune!
Li: Hmmm. I have to say I don't think I change my tune very often.
William: Well, I'm sure that's true but we don't generally use the phrase to talk about ourselves. We tend to always say: "You've changed your tune" or "You've certainly changed your tune." And we say it like this: "You've changed your tune".
Li: OK. And you have certainly changed your tune, William, about super jumbo hamburgers.
William: No ... I've always loved those things!
Li: No, trust me, Will. Hang on ... Here you go. Two weeks ago you said this: "I hate super jumbo hamburgers. The meat is like low-grade frozen cardboard, the cheese is like plastic and the tomatoes are like rubber. After I eat one I just want to curl up in bed." That's what you said!
William: Oh, right. Well, I guess they must have grown on me.
Li: What a change of tune, from William, the burger hater to William, the burger lover!