Whenever, upon an investigation made under the provisions of this title, the commission shall find any existing rate or rates or any regulation or practice whatsoever or any service, unreasonable or unjustly discriminatory, or any service inadequate, it shall so determine and by order fix, to the extent that it is within its power to do so, a reasonable rate, fare, charge, classification or joint rate as between like carriers, to be imposed, observed and followed in the future in lieu of that found to be unreasonable or unjustly discriminatory, or inadequate, as the case may be. All utilities to which the order applies shall make such changes in their schedule of rates, fares, charges or classifications as may be necessary to make the same conform to said order, where such order relates to rates, fares, charges or classification, and no change shall thereafter be made by any utility in such rates, fares, charges or classification, or joint rate or rates, or in the service or practice so ordered, without the approval of the commission.