I write a lot about transformation as something freeing and liberatory and thus generally depict it as a mostly painless process defined by relief and revitalization, but transformation as cathartic, exhausting agony is also pretty good.
the pain of change, the pain of the shell that imprisoned you being finally ripped away. cracks and pops as new limbs take shape, old ones change form, your very skull reshapes. A fierce migraine, the overwhelming disorientation of beginning to process new, changed senses. A scream of agony that turns into an inhuman, furious roar. An experience of overwhelming intensity - being finally shocked alive after a lifetime languishing, all but dead, in a cage of quiet unease.
And then, in the aftermath, you're laying on the floor, in a sort of calmed shock. Drawing deep, pained breaths through a long snout, slowly gaining the strength to move again. But the limbs you're slowly moving, they're changed, different. The prison is finally gone; burned away in the agony, revealing at long last only the real you.