Place yams in a large saucepan and cover with cold water. Cover and bring to a boil over high heat. Reduce heat to simmer and cook for about 25 minutes or until yams are tender. Drain and let cool to room temperature. Meanwhile, place apple juice concentrate in a small saucepan. Heat concentrate until it's reduced by half, remove from heat and add butter. Peel cooled yams and cut into ¼-inch slices. Place slices in an 8-inch square casserole and pour apple butter mixture over yams. Bake at 350° for 20 minutes.