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