Arceus will never change it's type with plates! Think about it. Are they really going to release 8 cards in a single set that can be only used on a single pokemon? Even one plate would be stupid if only one pokemon can use it. I don't think so. It would just be a waist of good cards slots where there could be some really much more usable trainers. Would you take the plates over cards like Bebe's, POV, Night Maintenance or other good draw power card or searching stuff. People would be so fed up with getting a plate in every booster and no Arceus to use them on (I imagine it would be a Holo Rare).
No. If it is released as a card, it will have a Poke-Power or Poke-Body, that will allow it change type. (Remember Kecleons and Ninetales MT? Why nor Arceus?) Or it will be plain normal type.
One good option is to release Arceus as a special promo card. As well as Sheymin could be. I could easily imagine Sheymin being released as a movie promo card.