(a) For the purpose of prescribing minimum qualifications for appointment of local social services commissioners, differentiated on the basis of their relative responsibilities, the social services districts of the State are hereby divided into four groups as follows:

(b) The caseload of each social services district on June 30th of each year, as determined by the commissioner, shall determine the group in which a social services district shall be, for the purpose of determining the minimum qualifications required by this Part for candidates to fill a vacancy occurring within one year after that date.