Except that once a Changeling chooses, they literally and permanently transform. They become either 100% Fairy or 100% Human.
Remember Meril (Lilly and Fix's Friend). She chose her Fairy side and turned into a troll.
Before I had assumed (Also I heard WoJ somewhere) That Lilly Chose to be Fairy when she became the Winter lady. I had assumed that was a neccessary part, since only a Faerie could bear the mantle. She went from Changeling to Fairy by the normal process of a Changeling choosing to embrace their Fairie half.
Lilly could have become a Fairy without the mantle.
Molly is 100% Human. The question is does the mantle turn her into a Sidhe (The "Fair Folk" species of Faerie that most of the Winter and Summer court nobles seem to be, as opposed to Trolls, Ogres, Centaurs, ect), or does she remain primarily Human.