All collective bargaining labor agreements hereafter entered into between the management of a utility and its employees or any craft or class of employees shall be reduced to writing and continue for a period of not less than one year from the date of the expiration of the previous agreement entered into between the management of the utility and its employees or if there has been no such previous agreement then for a period of not less than one year from the date of the actual execution of the agreement. Such agreement shall be presumed to continue in force and effect from year to year after the date fixed for its original termination unless either or both parties thereto inform the other, in writing, of the specific changes desired to be made therein and shall also file a copy of such demands with the state board of mediation, at least sixty days before the original termination date or sixty days before the end of any yearly renewal period, or sixty days before any termination date desired thereafter.