It means a thorough and dramatic change in something
This is what happens before the story begins in "The Cask of Amontillado", according to my imagination:
Fortunato had always been an arrogant man who loved to laugh at his friends' imperfections or mistakes in general. When Montresor first met him, he was amazed at how clever and generous that man seemed to be. However, such an impression quickly died out: Fortunato turned out to be a rude, unkind, bitter man. That was a huge disappointment for Montresor, who really treasured his friend, at least in the beginning of their friendship.
One day, the two friends were having a casual conversation, when suddenly Fortunato turned to Montresor and said to him: "I'm sure I'm better than you at anything; that's why you were so jealous whenever I came around that girl you fancied..." Montresor was shocked to hear that, to which he replied right away: "Jealous? What are you talking about?" Fortunato had a cynical smile on his face, and said next: "So you think I didn't notice how insecure you felt... Maybe you were afraid she would fall in love with me, not with you..." After hearing that, Montresor was oppressed with anger, and couldn't say a word since if he did so, he would probably burst out in tears of uncontrollable rage.
After having spent the following night in a state of agitated insomnia, Montresor had an idea: inviting Fortunato to go taste some wine in a place where no one would bother them and that would also be suitable for a sweet and irreversible revenge.
