I actually ended up doing what you suggested – but by animating the control points + adding a mask.
Shape 1:
– masking out shape 2. The initial shape is bigger than shape 2 (no masking effect in the initial state)
– animating individual control points to final shape (thus masking out shape one)
Shape 2:
– animating individual control points from initial shape to being outside the mask (shape one)
/Donleavy