In the original myths and legends of the Kingdom-Empire, particularly in the Verge, changelings were fae that were left in place of stolen mortal babies. Other than a few isolated cases, that bas proven to be a myth, and as such the term has gradually shifted to mean those creatures that are of mutable form, captable of taking on a variety of guises.
The general consensus among the Collegium is that changelings either have some undetected fae (or possibly draconic) ancestry, or have been exposed to some sort of potent magical contamination at an early age or in utero. In either case, changelings’ abilities vary widely and inconsistently, with many arguing that they are in fact not even a discrete group at all.
Most changelings appear to attempt to blend in, assuming an innocuous form and attempting to maintain it. Some are able to do this more easily than others, but being outed as a changeling almost inevitably leads to the being in question being ejected from any employment and family bonds in suspicion of being an imposter.
They are, of course, treasured as spies and courtesans by the elite, but rarely if ever in the open due to the intense stigma, and many a changeling in government employ has only been revealed as such on their death. It was widely spectulated, but never proven, that King-Emperor Francis III’s well-known lover Hibibeth Dio was in fact a changeling, which led to her “disappearance” immediately after Francis III’s death.