Vacancies in positions in a paid fire department shall be filled, so far as practicable, by promotions from among individuals holding positions in the next lower grade in the department. Promotions shall be based upon experience and by competitive written examinations to be provided by the firemen's civil service commission: Provided, That no individual shall be eligible for promotion from the lower grade to the next higher grade until such individual shall have completed at least two years of continuous service in the next lower grade in the department immediately prior to said examination and has completed the registered apprenticeship and certification program under article fifteen-a, chapter eight of this code: Provided, however, That completion of the registered apprenticeship and certification program as a requirement for promotion shall apply only to those firefighters employed since June 12, 1987. The commission shall have the power to determine in each instance whether an increase in salary constitutes a promotion.