I have a feeling that the forme changes will work something like how Shaymin changed forme (not including the "only stys that way when not Frozen/in the middle of night" factor). If we all remember, the Black/White Orbs were Key Items. If Kyurem changed forme via those, then first of all, it would prevent the player from transfering it to B/W, and second, you wouldn't be able to get rid of it, unlike with Giratina's Griseous Orb in Plat./GH/SS, if you accidently hit "trash" instead of "hold," it would be gone for good, since DP never "knew" of anything called the Griseous Orb (needless to say, you can transfer the Griseous Orb from BW from game to game now).