  It is known that the extremal Reissner-Nordstr\"om black hole possesses a
discrete conformal isometry that exchanges the black hole horizon with
infinity. It is also known that the Reissner-Nordstr\"om-de Sitter spacetime
posseses a similar discrete conformal isometry which exchanges the event
horizon of the black hole with the cosmological horizon. In this short paper we
will continue this line of thought and extend the conformal isometry of the
latter spacetime and give an unphysical interpretation of the negative root.
