The kind of story I would be thinking of is a version of "Cinderella," in which the newly crowned princess is asked by the prince to act as the "fairy godmother" to some other downtrodden young lady. Or Dorothy, after having been given the ability to move between the land of Oz and Kansas (and other powers), is asked to return to the land of Oz to take over for the old and dying (good) witch of the North.
The closest thing I can think of is the "Princess Diaries," where the 16-year old daughter of an American mother (Anne Hathaway) is asked to take over her deceased father's Monaco-like country of Genovia, and step into the shoes of the old queen mother (Julie Andrews).
Is there a "standard" story line of this kind, and if so, what would it be called?