  After pioneering gaseous detectors of single photon for RICH applications
using CsI solid state photocathodes in MWPCs within the RD26 collaboration and
by the constructions for the RICH detector of the COMPASS experiment at CERN
SPS, in 2016 we have upgraded COMPASS RICH by novel gaseous photon detectors
based on MPGD technology. Four novel photon detectors, covering a total active
area of 1.5~m$^2$, have been installed in order to cope with the challenging
efficiency and stability requirements of the COMPASS physics programme. They
are the first application in an experiment of MPGD-based single photon
detectors. All aspects of the upgrade are presented, including engineering,
mass production, quality assessment and performance.
  Perspectives for further developments in the field of gaseous single photon
detectors are also indicated.
