Why does the setting change during "The Cask of Amontillado” and what symbolism causes the change? How does it compare to the carnival setting in “William Wilson" (i.e., how, why, to what effect)?

The two stories by Edgar Allan Poe, "The Cask of Amontillado” and “William Wilson," are somewhat similar in structure. In both cases, there are narrators who are fundamentally evil. They were both born to wealthy noble families, but there is a sense that their families have degenerated both in external circumstances and moral nature. The stories themselves are framed as confessions, made toward the end of the narrators' lives, of some great evil deeds they have performed. In both cases, the carnival forms a backdrop to specific actions planned by the narrators.
In the case of "The Cask of Amontillado,” the occasion of the carnival provides enabling circumstances for Montresor's murder plot. First, it enables Montresor to ensure that no servants will be present in the house when he takes his revenge. Second, it enables him to contact and lure away Fortunato without drawing attention to himself. The main reason for the change of setting is literal and practical. It would be difficult to kill Fortunato in the middle of a crowd without getting caught. On a symbolic level, the carnival is a public space, in which competitions over wealth and prestige are played out in front of an audience. Montresor's hatred and Fortunato's foolishness are both part of this public world of display and appearances. The carnival is also a place of deception and false appearances. On a literal level, people are appearing in costumes, but on a deeper level Montresor is pretending to possess a benevolence and sociability he lacks in reality and Fortunato is feigning a sophistication and discernment he lacks. Once they leave the carnival, the masks fall off and the characters reveal their true nature.
The narrator of “William Wilson” attends a carnival at the end of the story, for the purpose of committing adultery with the young and beautiful wife of his host. As is the case in "The Cask of Amontillado,” the carnival is a place of deception, and under its cover, protected by the costumes of the masquerade, people can plot deceptive or criminal deeds. Just as Montresor lured Fortunato away from the carnival to murder him, so Wilson takes his double into an isolated antechamber to stab him.
Although the carnival setting in the "The Cask of Amontillado” is outdoors and everyone in the town is attending, while that of “William Wilson” is a private party, both are similar in being masquerades in which people on the surface are pretending to be someone else (by wearing disguises) but at the same time, beneath those surface disguises, revealing their true inner natures.