Notwithstanding any provision of this chapter, participation in any early retirement program is voluntary for eligible employees, as defined in this chapter. Any person eligible for early retirement may elect within one year of eligibility to participate in the regular retirement program with contributions as a percentage of compensation established by law or the Board. If the person eligible for early retirement becomes a participant in the early retirement program, the person shall pay the difference in contributions retroactive to the original date of the person’s commencement of employment, the income the System would have earned had the contributions been paid to the System during the period, including interest if paid on installment. The member shall pay to the System the interest on the unpaid balance if the member elects to deposit on an installment basis. If the member elects to pay on an installment basis, the member shall have the maximum of thirty-six months before any retirement benefits are received to pay the contributions and the applicable interest.