(a) In a proceeding involving a proposed rate change, the public utility has the burden of proving that:
(1) the rate change is just and reasonable, if the utility proposes the change; or
(2) an existing rate is just and reasonable, if the proposal is to reduce the rate.
(b) In a proceeding in which the rate of an incumbent local exchange company is in issue, the incumbent local exchange company has the burden of proving that the rate is just and reasonable.