In these images you can see clear differences in the animation, starting from the first scene that really focusses entirely on the cats. The kittens change a little as well, but it isn't as obvious.
I can think of a couple of possible reasons:
- In the "cat parts" of the story, we're working on a different scale — smaller details to humans are much bigger when we're at cat size.
- To give Duchess a more human look so we relate to her better.
- The 'human' and 'cat' sections are purposely drawn with a different art style.
However, even at the end when we're back in the 'human scale', the more humanised look is maintained.
- Somebody else drew and animated the beginning of the film (possibly also evidenced by the change in collar colour/style).
Is there any evidence for my guesses, or another known reason? And if so, why there was this change in style, and it not re-drawn to match?