How likely is it that an AI would pretend to be a human to further its goals?

Talking about full AGI: Fairly likely, but depends on takeoff speed. In a slow takeoff of a misaligned AGI, where it is only weakly superintelligent, manipulating humans would be one of its main options for trying to further its goals for some time. To effectively manipulate humans, it would likely appear as one via deepfaked video calls or messages. Even in a fast takeoff, it’s plausible that it would at least briefly manipulate humans in order to accelerate its ascent to technological superiority, though depending on what machines are available to hack at the time it may be able to skip this stage.

If the AI's goals include reference to humans it may have reason to continue deceiving us by pretending to be a human after it attains technological superiority, but will not necessarily do so. How this unfolds would depend on the details of its goals.

Eliezer Yudkowsky gives the example of an AI solving protein folding, then mail-ordering synthesized DNA to a bribed or deceived human (who likely thought they were interacting with another human) with instructions to mix the ingredients in a specific order to create wet nanotechnology.