The term Changeling is used loosely to describe a wide variety of cyborgs which have biological bodies but have an artificial processor array instead of a biological brain. The first Changelings emerged in the Interplanetary Era, when modosophont-level artificial brains were too large to fit inside the skull if a human or other biont; instead these early changelings incorporated a teleoperation rig inside the subject's skull instead, so that a much larger processing array elsewhere could take control of the body.
By the Late Federation Era, human-level artificial brains could be accommodated within a human skull, including all the necessary power and support systems necessary for the Changeling to maintain this arrangement permanently. Since that time a number of very different clades have adopted this configuration, such as Homo Economicus and the Bright Cats, although there are many significant differences between the psychoware and interfacing in each case.
Some sophonts choose to adopt the Changeling lifestyle because this allows them to retain their familiar biological body while transferring their mindstate into a faster and more durable substrate; others choose to transfer their mind into an entirely different biological body, using translation protocols to help them adapt to the new sensorium and the unfamiliar physical nature of their new home.