The transitions from the incoherent noise to the coherent soliton have been fully revealed in ultrafast lasers. However, the soliton transformation between different coherent states, termed as soliton metamorphosis, remains an attractive yet uncharted territory. Here, we reveal the ultrafast dynamics of the soliton metamorphosis via single-shot spectroscopy in a specially designed fiber laser capable of emitting fast-switchable dissipative solitons and stretched pulses. It is demonstrated that the soliton metamorphosis is a consecutive evolution process including the self-phase modulation stage, pulse split stage, and transient stretched pulse stage. Particularly, the long-period pulse breathing and the spectral period doubling appear in the forepart and middle part of the last stage. The metamorphosis dynamics and soliton properties are substantiated by numerical simulation based on a three-step model. This work not only unveils the transient evolution physics of the pulse in soliton metamorphosis, but also provides a simple and effective way to control operations of ultrafast lasers.