I would recommend 'Looking for Alaska' by John Green. The events in the story lead to a shift in the main character's view of life and death, love and loss. 'Thirteen Reasons Why' by Jay Asher also fits the bill. The tapes left by Hannah Baker cause a shift in the way Clay Jensen views his actions and the world around him, although this book has been controversial due to some of its themes.
A really good shift young adult novel is 'Eleanor & Park' by Rainbow Rowell. The relationship between Eleanor and Park causes shifts in their self - perceptions and how they interact with the world. Also, 'The Hate U Give' by Angie Thomas is an important one. The main character, Starr, has a major shift in her identity as she becomes an advocate after witnessing the shooting of her friend, which also involves a shift in her understanding of social justice.
One great shift young adult novel is 'The Perks of Being a Wallflower' by Stephen Chbosky. The main character, Charlie, experiences a significant shift in his self - understanding and his relationships throughout the story. Another is 'Speak' by Laurie Halse Anderson, where the protagonist goes through a shift as she finds her voice after a traumatic event.