The Changeling is a scout unit that permanently disguises itself as an enemy unit when it gets near an enemy . Once a Changeling has disguised itself, it can not go back to the Changeling model or disguise itself as a different player's unit. Disguised changelings are not automatically targeted by enemies. Changelings can not be loaded into transports. Cavez described it in detail as follows.
After a question regarding the visuals, specifically the Marine's shields, Cydra found out that the Changeling will take the appearance of the upgrades of the unit it disguises, such as the Marine's shields, but not effects from them, such as the Zealot's extra speed from Charge.src
In the latest Blizzcon 09 build, the Changeling received new visuals, but a huge health decrease in the process.
In November 2009, Blizzard released "Changeling", a short story by James Waugh. It includes a short video showing the Changeling in-game.