Forme-changing items should be de-activated when obtaining them.

[tsuchinokoReal]

Adding Forme-changing items to a Pokémon with one already equipped marks them as "activated", although the item does not affect the Pokémon. This causes confusion when trying to switch formes for the first time. A user has to deactivate all forme-changing items, even ones that are not actively affecting the Pokémon, to enable another one.
This affects Necrozma, Genesect, Ogrepon, Deoxys, and Calyrex. It will affect Arceus and Silvally in the future when their abilities are implemented.