Its completely legal to move when the other side is serving.
Originally Posted by himynameisminh
I played a duo in doubles that didn't do this exact thing; however, one of the players, when awaiting the return at net, would strafe in a perimeter of the service box as soon as I or my partner was getting ready to serve. It irked us A LOT, but we didn't call a hindrance. His partner did nothing of the sort.