Employees still in service whose annuities were fixed at age 70 prior to July 1, 1988. For all employees who have not withdrawn from service or retired, whose annuities were fixed under prior law at age 70, prior to July 1, 1988, contributions and service credits shall be resumed on January 1, 1990. However, no contributions or service credits shall be permitted from the time that annuities were fixed to January 1, 1990.