It's easier for the average person to shapeshift into the form of a body their soul has previously inhabited. It's like a template of sorts. Preferably they'll use the most recent shape, since their souls remember those better than older ones, which insures the stability of the transformation and makes it easier to learn how to move the new body. In Jason's case that's be a juvenile cypro that can't even fly yet, however he's still stronger and faster in that form.

Jason on Toyhouse