"变卦" (biàn guà) means that someone suddenly changes his/her mind on something that has already been decided. People usually use this word to express dissatisfaction with the person who has changed.
1. wǒ tǎo yàn biàn guà, wǒ bú huì fù nǐ yì bǎi měi yuán. biàn guà shuō míng nǐ sā huǎng.
I hate to go back on my word, but I won't pay you $100 afterall. Going back on your promise makes you a liar.
2. zuó tiān shuō dé hǎo hao de, zěn me biàn guà le ?
Yesterday you agreed. What made you change your mind?
The article is translated and editted by Chinlingo. Please indicate the source for any use, reproduction or transfer.