Section 32. The department shall prescribe the forms for the annual returns to be made to it by the several kinds of common carriers and may from time to time make changes and additions in any such form; provided, that changes therein or additions thereto requiring any alteration in the method or form of keeping the accounts of such common carriers shall not be effective until after notice thereof shall have been given, at least six months before the beginning of the year or other period for which such changes or additions are prescribed, by the department to the common carriers affected thereby. The annual returns required by law to be made to the department shall be returns for the year ending on December thirty-first, and shall be transmitted thereto, upon blank forms to be furnished by the department, on or before March thirty-first following, or such subsequent date as in any case, for good cause shown, the department may fix. If a return is defective or appears erroneous, the department shall forthwith order the common carrier to amend it within fifteen days. The original of each return or amended return, subscribed and sworn to as required by law, shall be preserved in the office of the department.