I recently found the Pixie Plate which boosts Fairy Type Moves, but I remembered Arceus can change types depending on which Plate you give it. Does this mean Arceus can change to a Fairy type now?
Yup. Pixie Plate will do the same thing all other plates do; boost type specific moves by 20% and change Arceus and Judgement to Fairy type.
Serebii - Pixie Plate
Yes, with the Pixie Plate he can turn into a fairy type. He turns into every type.