Research Article: The Swiss Light Source Upgrade to a Diffraction-Limited Storage Ring - Philip R. Willmott, Gabriel Aeppli, Alun Ashton, Christoph Bostedt, Oliver Bunk, Marco Calvi, Yasin Ekinci, Uwe Flechsig, Romain Ganter, Terence Garvey, Markus Jörg, David Just, Roland Kobler, Frithjof Nolting, Luc Patthey, Claude Pradervand, Jörg Raabe, Leonid Rivkin, Benedikt Rösner, Thomas Schmidt, Andreas Streun, Friso van der Veen, and Hans Braun, Paul Scherrer Institute