I'm still reading the novel . . . or listening to the audio book really . . . and I think the term "changeling" refers to severely genetically enhanced/altered animals, not humans. Although one of the Celestial characters dismissively refers to the Uranic humans as "mere changelings" . . .
We've got animals that are uplifted to near sentience and human-like intelligence, with weird abilities to serve specific purposes. Early on, a police character has two super-dogs as partners, highly intelligent, big, strong, and incredibly agile. Later our heroes discovers a race of centaur-like changelings who are designed to remember, to hold memory! And being exploited by having brain fluids drained to make a narcotic drug . . .
The Uranic humans are the humans put in charge of the human words by the Celestials . . . think noble families . . . and gifted with the same ability Celestials have to interface with computer networks. But other than that, seem to be normal humans.