How do I use a chengyu (idiom) to describe a "one extreme to the other" kind of transformation?

Question

In my story, 孔琳's behavior has changed from ultra-depressed to ultra-optimistic in a matter of moments (she has an unstable personality). Her friends (陈叶 and 胡灵阳) are staring at her in amazement, and I want to write...

现在陈叶和胡灵阳在一起站着，惊喜地盯着孔琳[chengyu]的变化。

I want to describe this change using a chengyu, but the closest I found was 以耳代目 and 以耳为目 which are variants. It's not bad (it means something like "substitute ears for eyes"), but I don't think it works grammatically above. I'm also unsure that the context works here; perhaps it refers to blind people.

Question: How do I use a chengyu (idiom) to describe a "one extreme to the other" kind of transformation?

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English derived from above Chinese; only to be used "in case of emergency" (i.e., if the Chinese is incomprehensible):

_{^{Now Chen Ye and Hu Lingyang [are] standing, surprisedly staring [at] Kong Lin's from-one-extreme-to-the-other change.}}

Answer 1

Depend on what the emphasis is, there are quite a few idioms you can use in this context.

Some suggestion:

判若兩人 (like a different person)

前後判若兩人 (like a different person from before)

大起大落 (up and down to the extreme/ high and low to the extreme)

峰迴路轉 (surprising, unexpected)

前後反覆 (change completely from before/ turn 180 degree)

一百八十度轉變 (turn 180 degree)

If you are looking for an idiom 前後判若兩人 fit in this context the best, but 一百八十度轉變 itself is a common phrase

Answer 2

忽喜忽忧 is probably what you need. It means 时而欢喜，时而忧伤。

For example:

投资人的心情随着股价涨跌，忽喜忽忧，变化莫测。
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Answer 3

To add to the collection: 反复无常：Changed unpredictably to something that is in an opposite direction.

e.g. The weather can go from one extreme to the other. == The weather can go 反复无常