Section 18.  Except as inconsistent with this Part, the provisions of Article XIV, Section 15.1 of the Constitution of 1921 are retained and continued in force and effect as statutes.  By law enacted by two-thirds of the elected members of each house, the legislature may amend or otherwise modify any of those provisions, but it may not abolish the system of classified civil service for such firemen and municipal policemen or make the system inapplicable to any municipality having a population exceeding thirteen thousand according to the latest decennial federal census or to any parish or fire protection district operating a regularly paid fire department.  However, in a municipality having a population exceeding four hundred thousand, paid firemen and municipal policemen shall be included if a majority of the electors therein voting at an election held for that purpose approve their inclusion.  Such an election shall be called by the governing authority of the affected city within one year after the effective date of this constitution.