No change shall be made in any schedule required to be filed pursuant to § 56-236, including schedules of joint rates, except after 30 days’ notice to the Commission, in such manner as the Commission may require, and to the public, in such manner as prescribed in § 56-237.1, and all such changed rates, tolls, charges, rules, and regulations shall be plainly indicated upon existing schedules or by filing new schedules in lieu thereof not less than 30 days prior to the time the same are to take effect; provided, that the Commission may, in particular cases, authorize or prescribe less time in which changes may be made; and provided further that, in the case of water companies, the notice to the public shall set forth the proposed rates and charges.