(a) Except where otherwise indicated, this Part will not be applied retroactively to existing installations insofar as design, fabrication, installation and testing are concerned.
(b) However, all the provisions of this Part will be applicable to any upgrading or reconstruction. Upgrading is also specifically subject to the provisions of sections 255.552 through 255.557 of this Part.
(c) Existing installations will conform to the requirements of this Part's predecessor in effect at the time of installation.
(d) Provisions of this Part are applicable to the operation and maintenance of existing and new installations.