I had already done that. The issue is that it doesn't update the collider at runtime.
And now I have an even worse issue, which is that I can't change the animation at all at runtime :-/
Not sure if it's a bug - I don't think I changed anything!
I'm calling it as follows (My animations are named "0", "1", "2", etc., and all the animated sprites are stored in an array called "_sprites"):
for (var i : int = 0;i < _sprites.length; i++) {
var anim:int = Random.Range(0,4);
_sprites[i].GetComponent(tk2dAnimatedSprite).Play( anim.ToString() );
}