No change shall be made in any schedule, including schedules of joint rates, except upon 30 days’ notice to the Public Utility Commission. All changes shall be plainly indicated upon existing schedules, or by filing new schedules in lieu thereof 30 days prior to the time they are to take effect. However, the commission, for good cause shown, may allow changes without requiring the 30 days’ notice by filing an order specifying the changes to be made and the time when they shall take effect. [1987 c.447 § 15]