This science fiction series with a fantasy flavor is about the immortal shapechangers. Not werewhatevers, but a few rare people with the ability to change their own bodies by wishing for a change.

They are so good at this that they need never age, or get sick, and can heal wounds very quickly. They are not bestial, less than human, but more human. And can improve not only their bodies but their brains – VERY slowly and cautiously.