Metamorphosis. Meta - beyond; morph - form; osis - process or change. A creature that has a different shape and functions as an adult than it did as a juvenile is said to have metamorphosed.
Metamorphosis may be partial (as in gilled tadpoles becoming air-breathing frogs) or complete, as in a caterpiller becoming a moth. Higher animals generally do not experience metamorphosis, their juveniles gradually changing into adults.