Did this a while ago for  panahinuva

As Millianna and the toy exchanged essences, both began to change. The toy grew larger, its grey coloration giving way to maroon and yellow. It sprouted stubby limbs and its body separated out from its head, growing comically oversized. Its tail thickened and fluffed up, while its ears shrank, becoming triangles sticking up over its head. Its nose shrank into a small black dot and two red stripes appeared on either side of its face, transforming it into a cat plushie.

Millianna, still mesmerized, shrank, down to about 4' 5". Grey fur spread across her body, starting at the grey glow and moving outwards. Her face began to expand into a small muzzle, nose flattening and turning red as it did so. Her hair ears merged back with her hair and her regular ears moved upwards, expanding and becoming overlarge circles above her head. As the grey fur reached her chest, it began to expand, doubling, then tripling, so they were even bigger than her head, jiggling with every move she made. Her tail fused with her tailbone, then grew longer and thinner, fur vanishing off of it as it grew. Finally, her front teeth grew longer and larger, poking out from under her lip and giving her a very visible pair of buck teeth.

She clutched the cat toy to her protuberant chest as her eyes faded back to normal. "Oh Mr. Meow, I don't know what I'd do without you." The mouse girl squeaked, sounding like Millianna had inhaled helium. "I'm so glad that someone sent you back to me."