Abruptly the narrator switches mood from boredom and frustration to excitement. To what does she attribute this change? What new aspects of the wallpaper does she discuss?

In the famous Gothic short story "The Yellow Wallpaper" by Charlotte Perkins Gilman, a young woman has been taken to a colonial mansion for the summer by her husband, a physician named John. He has diagnosed her malady as "temporary nervous depression—a slight hysterical tendency" and prescribes medication and rest. For her convalescence, he chooses a former nursery with stark yellow wallpaper where she spends much of her time. She finds the color repellent and begins to imagine and envision a woman trapped behind the wallpaper.

The narrator is, at first, bored and frustrated because her husband is so condescending towards her. He won't let her do anything except rest, and he won't allow her to move to another room when she complains about the wallpaper because he feels it's wrong to give in to her moods.

The switch from boredom to excitement happens as the narrator becomes more aware of the woman she imagines trapped behind the wallpaper and begins to see her more clearly. She is concerned that someone else might detect the woman, and she is determined to learn more about her before that happens. Her husband thinks she is recovering, and she writes: "I had no intention of telling him that it was because of the wallpaper—he would make fun of me." She peels off the wallpaper with the intention of freeing the woman, until in the end, she becomes the woman that she thought she was setting free.

We can see then that the answer to both parts of your question, the narrator's mood and the new aspects of the wallpaper, are related. They both have to do with the fantasy woman that the narrator imagines she finds behind the wallpaper.