Definition
change your tune : To be used in a context when a person changes his opinion completely, especially when it brings him an advantage.
change your tune : 心変わりする、態度や意見をころっと変える。
Example
DIdn’t you say before that learning the English language was hard? What made you change your tune? – Language exchange did, it is so much fun.
前は英語の勉強は難しくて大変って言ってなかった?何でそう思うようになったの?- 言語交流のおかげだよ、すごく楽しいんだ。