The Changeover — Margaret Mahy

The change was for life.

Laura had noticed the looks Sorenson Carlisle gave her at school. They were looks that meant more than just friends. And even though he hardly spoke to her, she knew there was something very special about him. She knew he was a witch.

So when Laura's brother Jacko falls deathly ill, Laura must go to Sorry -- for she can see Jacko is possessed. And Sorry tells her there is only one way to save him: to give up her mortality, and "changeover" to what he is himself.

"Go while you still can," Sorry warns her. Laura will not listen. He offers her life, death, or the supernatural -- but the choice she makes must be her own.