a demeanor (attitude, tune) undergoing an instant change for the worse (in colloquial usage) 意味

• a demeanor (attitude, tune) undergoing an instant change for the worse (in colloquial usage)
  君子豹変
  くんしひょうへん
  

• instant change:    瞬時｛しゅんじ｝［即座｛そくざ｝］の変化｛へんか｝Instant changes aren't realistic. 瞬時［即座］の変化は現実的ではない。
• change for the worse:    悪化｛あっか｝する
• to change for the worse:    to change for the worse化けるばける