Notwithstanding the provisions of this chapter, participation in the early retirement program shall be voluntary for eligible employees as defined by this subchapter. Any person eligible for early retirement may elect to participate in the regular retirement program with contributions established at a rate of compensation determined by the Board. Should such person eligible for early retirement become a participant in the early retirement program, they shall pay the difference in contribution retroactive to December 29, 1986 or for five years, whichever is greater, or to the original date of their commencement of employment.