Metamorphosis was a change undergone by various lifeforms.
Caterpillars underwent a metamorphosis into a butterfly, as did giant maggots into giant flies. (TV: The Green Death) Solonians experienced "accelerated genetic metamorphosis" seasonally. (TV: The Mutants) The Wirrn also underwent metamorphosis. (TV: The Ark in Space)
The newly regenerated Second Doctor compared his change to a butterfly leaving its chrysalis. (TV: The Power of the Daleks) Both Mawdryn and the newly regenerated Sixth Doctor also compared regeneration to metamorphosis. (TV: Mawdryn Undead, The Twin Dilemma)