Metamorphosis

You may be looking for the comic story of the same name or the Big Finish anthology of the same name.

Metamorphosis saw Solonians take on an insectoid form in the summer. (TV: The Mutants)

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)

Rutan scouts were specially trained in metamorphosis techniques. (TV: Horror of Fang Rock)

Princess Astra of Atrios, realising that she was the sixth segment of the Key to Time, referred to her "metamorphosis". (TV: The Armageddon Factor)