How to change registration point dynamically as3

function setRegPoint(obj:DisplayObjectContainer, newX:Number, newY:Number):void {
    //get the bounds of the object and the location
    //of the current registration point in relation
    //to the upper left corner of the graphical content
    //note: this is a PSEUDO currentRegX and currentRegY, as the
    //registration point of a display object is ALWAYS (0, 0):
    var bounds:Rectangle = obj.getBounds(obj.parent);
    var currentRegX:Number = obj.x - bounds.left;
    var currentRegY:Number = obj.y - bounds.top;
   
    var xOffset:Number = newX - currentRegX;
    var yOffset:Number = newY - currentRegY;
    //shift the object to its new location--
    //this will put it back in the same position
    //where it started (that is, VISUALLY anyway):
    obj.x += xOffset;
    obj.y += yOffset;
   
    //shift all the children the same amount,
    //but in the opposite direction
    for(var i:int = 0; i < obj.numChildren; i++) {
        obj.getChildAt(i).x -= xOffset;
        obj.getChildAt(i).y -= yOffset;
    }
}
book.addEventListener(MouseEvent.CLICK, bookClicked);
function bookClicked(event:MouseEvent):void {
    book.scaleX += .1;
    book.scaleY += .1;
}

//set the registration point for the book object in the middle instead:
setRegPoint(book, book.width / 2, book.height / 2);