Changelings were individuals controlled by the Graske that mimicked a lifeform and sent the real one to Griffoth. They looked almost exactly like the originals, but their eyes flashed a greeny-gold colour occasionally, hinting at their true disposition. Changelings appeared to behave in just the same way as the originals, so that hardly anybody suspected them. The Graske needed to keep the originals alive in order to sustain the changelings. A Graske created Changelings for a Slitheen, a street urchin, a mum and a dad. (NOTVALID: Attack of the Graske)
The player could choose from two outcomes with his or her remote control while playing Attack of the Graske. They could return the originals the changelings replaced to their rightful place, or freeze the chamber containing the originals, stopping the Graske but leaving the changelings on Earth.
The fact that the Graske needed capture individuals and keep them alive to sustain the changelings is similar to the original method used by the Zygon to take on the forms of others before they gained the ability to take on the forms of others without the need to hold a subject captive to maintain their form.