The provisions of this act shall not be construed as affecting any permit or authority heretofore granted or given pursuant to law for the construction of any dam, or for the changing or diminution of the course, current, or cross-section of any stream or body of water; but the holder of any such permit or authority may apply for a limited power permit or a limited water supply permit under this act, and, if and when any such applicant is granted the same, he shall have all the rights and be subject to all the duties conferred or imposed by or under this act.
32 P.S. § 596