The Swiss authorities have informed CERN that a person who has exchanged a C permit for a DFAE "carte de légitimation" or a Ci permit is still entitled to apply for naturalisation after 1 January 2018, the date on which the new law and ordinance on Swiss nationality came into force (cf. Bulletin article of 9 April 2015, https://home.cern/fr/cern-people/official-communications/2015/04/new-law-swiss-nationality).
The person concerned must, of course, meet the other requirements imposed by the federal and cantonal legislation in the place of residence. Furthermore, the person must supply a copy of their DFAE "carte de légitimation" or Ci permit, indicating the date on which the C permit was exchanged. As a general rule, minors are included in the naturalisation application, but, when they reach the age of 12, certain requirements are considered separately.
Persons who have exchanged a B permit for a DFAE "carte de légitimation" or a Ci permit do not meet the requirements to apply for naturalisation.