Don't make too much out of positions. Doc really only sees two positions: "bigs" and guards.
As to who defends opposing centers, well, it doesn't matter all that much because there are only a handful of"true" centers left that would be too big for KG. But I think you can expect KG to be on the floor with a banger like Wilcox or Bass most of the time (who can both guard centers).
I think this move is directly related to playing Miami. Heat's best lineup is Bosh and Haslem down low. KG and West match that.