There are very rare exceptions to this "permanency"-for example, metamorphosis enables nothlits to regain their original form, as when Cassie is trapped in the body of a caterpillar but returns to human form after becoming a butterfly.
There are very rare exceptions to this "permanency"-for example, metamorphosis enables nothlits to regain their original form, resetting the morphing clock. In "The Depature", Cassie is trapped in the body of a caterpillar, but is able to morph again when she becomes a butterfly.