Animating CALayer's shadowPath property

Firstly, you did not set the animation's fromValue.

Secondly, you're correct: toValue accepts a CGPathRef, except it needs to be cast to id. Do something like this:

theAnimation.toValue = (id)[UIBezierPath bezierPathWithRect:newRect].CGPath;

You'll also need to set the shadowPath property of the layer explicitly if you want the change to remain after animation.