to either the Civil Service Retirement System under chapter 83 of title 5 (in this section referred to as CSRS ) or the Federal Employees Retirement System under chapter 84 of title 5 (in this section referred to as FERS ) on the day immediately preceding the date of transfer shall elect either to— (i) retain the employee’s coverage under either CSRS or FERS, as applicable, in lieu of coverage by the Corporation’s employee benefits system established under section 90316(c)