A written order to the contractor signed by the owner and the architect, issued after the execution of the contract, authorizing a change in the work or an adjustment in the contract sum or the contract time as originally defined by the contract documents; may add to, subtract from, or vary the scope of work. A change order may be signed by the architect alone (provided he has written authority from the owner for such procedure and that a copy of such written authority is furnished to the contractor upon request), or by the contractor if he agrees to the adjustment in the contract sum or the contract time.