(b) If a residential customer becomes eligible for a lower rate because he or she changes the character of the service received and if excess payment resulted from the failure of the customer to notify the utility or municipality providing service of a change in the character of service received or from the failure of the utility or municipality to change the rate after such notification by the customer, such a customer will be entitled to such lower rate from the date the change occurs but such a customer shall not be entitled to such lower rate for more than twenty-four months prior to the giving of actual notice to the utility or municipality providing service.