Vacancies in positions of correctional officer shall be filled, so far as practicable, by promotion from among persons holding positions in the next lower grade. Promotions shall be based upon merit and fitness, to be ascertained by competitive examinations to be provided by the civil service commission, and upon the superior qualifications of the persons promoted, as shown by their previous service and experience: Provided, That, except for a correctional officer appointed jailer pursuant to the provisions of section two, article eight of this chapter, no person shall be eligible for promotion from the lower grade to the next higher grade until such person shall have completed at least two years' service in the next lower grade: Provided, however, That notwithstanding the provisions of section one of this article, any correctional officer occupying the office of jailer pursuant to the provisions of section two, article eight of this chapter in any such county on the effective date of this article, or thereafter appointed to such office, shall, except as hereinafter provided in this section, be and shall continue to be entitled to all of the rights and benefits of the provisions of this article, except that he or she may be removed from such office of jailer without cause and the time spent by such person in the office of such jailer shall be added to the time, if any, served by such person during the entire time he or she was a correctional officer of such county prior to his or her appointment as jailer, and shall in all cases of removal, except for removal for just cause, retain the regular rank within said sheriff's office which he or she held, if any, at the time of his or her appointment to the office of jailer or which he or she has attained, if any, during his or her term of service as jailer. The provisions of this section shall be construed to apply and to inure to the benefit of all persons who have ever been subject to the provisions of this article. The commission shall have the power to determine in each instance whether an increase in salary constitutes a promotion.