Disney does not own the rights to Wicked, Universal does. In fact, Disney’s Oz The Great and Powerful has a quite different background in store for the Wicked Witch of the West. Also, keep in mind that the slippers were only ruby in the film version of The Wizard of Oz, while they are actually silver in the original book. Unless Disney wants to pay royalties to Turner, who owns the film, I think there’s more of a chance that the slippers will be silver on Once, if shown at all.