Since it’s all 2d and flat, why not use shape layers? First convert your AI layers to shape layers under the Layer menu – Layer > Create Shapes from Vector Layer – then animate the paths of your shapes exactly like you would with masks. Since you’ve got the shape in 3 states, I’d make a keyframe on the paths of the 2nd state, then copy them to the paths of the first state like 30 frames ahead in time (after setting keyframes on the first state of course), then do the same with the paths from the 3rd state. If you get some weird warping and rotating going on it’s usually because the first vertex points aren’t in the same place. Just scrub through the animation and watch where the vertex point is and make sure to set the first vertex as the same from one keyframe to the next.
If you want, I could mock this up for you and you could refine it. I love stuff like this.
Jason Jantzen
vimeo.com/jasonj