Mass isn't conserved. The sun operates by destroying matter, as do atom bombs. Energy, which matter is a form of, is conserved. The violation isn't the turning into a cat, it's the lack of a subsequent mushroom cloud.
Well... To be fair, the added matter could be shifted into a higher dimension and detached from the cat. You would then see a small cat with the expected mass of a small cat.
Plus, if you discharge the mass, you will have to somehow reclaim it when turning back into an human. Destroying the Earth wasn't a good idea to beging with, but freezing it doesn't sound better.
