Section 21. If either party is dissatisfied with the annual compensation established by proceedings upon a civil action under this chapter or corresponding provisions of earlier laws, a new action may be commenced for the increase or diminution of such compensation or for ascertaining the gross amount of the damages, and the action shall be conducted substantially in the manner provided for an original action; but if a plaintiff has declined to accept gross damages which have been awarded to him, they shall not be again assessed within ten years thereafter.