Changeling can mean one of two things, both associated with Oberon's Children.
1. "Changeling" is a term often used for "shape-shifter," a skill that Oberon's Children are particularly good at. Thus, Puck is often called a "changeling".
2. In earlier times, changelings were what the faerie children secretly exchanged for human children were called. Morgana le Fay and Nimue are the best-known of these.  Presumably, Oberon's Law has caused the practice of changelings of this nature to become discontinued in the Gargoyles Universe.
Real World Background
The second definition of "changelings" is indeed the correct one, according to actual faerie-legends. In these tales, the faeries would often steal away human babies, replacing them with their own kind. Sometimes these would be faerie infants who had failed to thrive, while more often, they were elderly faerie-folk who were disguised as babies, sent to the human world to live an easy life, being catered to by their "parents." Quite likely, these stories arose (at least, in part) to explain infant paralysis and other such diseases among babies.
In Shakespeare's A Midsummer Night's Dream, Oberon and Titania quarrel over the custody of a human child swapped for a changeling, a young Indian boy whom both Oberon and Titania wish to raise. (Greg Weisman has mentioned a private theory that this Indian boy might actually have been a son of Oberon by a mortal woman.) 
Changelings of the second variety never appeared in the televised episodes of Gargoyles, although Titania does indeed attempt to kidnap Alexander for the Gathering; the term is only used in the first (and strictly speaking, inaccurate) sense. ("The Gathering" Part One) 
- Changeling at Wikipedia, the Free Encyclopedia