In any case where the agency has heretofore adopted and established a classification or standards for any waters as then provided by law, the agency, at any hearing held pursuant to the provisions of this section for the purpose of modification, alteration, or amendment of such classification or standards or the adoption and establishment of any classification or standards for the same waters or any part thereof as required by sections 115.41 to 115.53, may receive and consider for any such purpose any testimony received at such previous hearing, as reported in the stenographic transcript thereof, and any exhibits received at such previous hearing, which are relevant, with like force and effect and subject to like objections, if any, as if such testimony or exhibits had been produced at the hearing hereunder, together with any further testimony or exhibits which may be submitted and received at the hearing hereunder.