What Would We Do Without Jill, Humanomorphs, Book 6 by M. D. Spenser

Jill is a pretty, bright girl whose mother makes her feel very unsure of herself. Jill's friends all tell her that she has everything going for her and that she should ignore her mother's comments.But Jill still wants to be like Molly, her best friend. She wishes it so hard that, briefly, she actually turns into Molly! This is when the two girls discover Jill has an incredible ability to morph into anyone she wants to be. That talent soon becomes a matter of critical importance for Jill. Molly and several other classmates attend a classical concert one afternoon. A gang of crooks, looking for hidden loot, takes over the concert hall and holds prisoner the orchestra and the entire audience of adults and kids. But Jill uses her great morphing skills to confuse the would-be killers - and through many clever morphs and some good common sense, Jill is able to save everyone from being killed by the crooks!