A changeling’s mien almost never changes so far from the changeling’s original form to make him unrecognizable. He may grow or shrink a few inches, but he is very unlikely to gain or lose a foot or more in height. Hair and eye color may change, and facial features may subtly shift, but the appearance of the stolen human is never completely overwritten.
It’s said that it’s possible to change completely, but that those who do never find their way home, having lost the ability to recognize their mortal selves. The changes are physical, but do not appear to be genetic; DNA analysis cannot determine a “goblin genome,” for instance. It also seems impossible for a changeling to pass on his fae qualities to any offspring — not that the Lost have much by way of fertility after the change, that is.
Upon returning to the mortal world, changelings may be relieved to find that the physical manifestations of their fae mien do not reveal themselves to humans. Changelings are protected by an enchantment that hangs over all things fae, concealing them from mortal eyes. This Mask, as they call it, may seem a small boon compared to the myriad other challenges they face in the mortal world, but it is a boon none the less.
Their miens, however, have not disappeared or faded upon leaving Faerie. Other Lost will see changelings quite clearly — as will the True Fae, should they come across the changeling. Likewise, should the changeling venture into the Hedge or Arcadia proper, he will find that everyone, including humans, sees his fae mien and not his human guise.