The Changelings are Trolls that were altered by the Gumm-Gumms so they can assume human forms. They send them to the human realm as spies, supplanting human babies from an early age. Changelings are immune to the effects of daylight regarding if they are in their human forms or troll forms.
The Changelings are Trolls who were abducted by the Gumm-Gumms from their youth, who did "something horrible" to change them. Unlike other Trolls, Changelings demonstrate a more humanoid appearance, such as a mesentage between human and Troll. By connecting with a specific human, his or her Familiar, the Changelings are able to assume the appearance of that human when they wish. In their human forms, the Changelings also appear to be immune to the effect of sunlight just like Stalklings thus, making them the perfect spies for the Gumm-Gumms. However, they can be revealed if you touch them with a gaggletack in the face (an iron horseshoe).
The Gumm-Gumms use Fetchs to sequester human babies and replace them with Changelings. The Changelings adapt perfectly to the human world, being able to be any person. It is known that there are currently hundreds of Changelings around the world, as well as hundreds of baby Familiars in the Darklands. They are organized under a group known as The Janus Order.
The Changelings are seen as inferior beings by the other Trolls, referring to them as "impures".