5 CFR 839.802: If I was in CSRS during my qualifying retirement coverage error, paid into the Fund more than I would have paid as a CSRS Offset, Social Security-Only, or FERS employee, and end up retroactively in one of those retirement plans, will I get a refund of the excess I had withheld from my pay?

CSRS Offset and FERS require employees to pay Social Security taxes in addition to retirement deductions. When you are retroactively changed under the FERCCA to CSRS Offset, FERS, or Social Security-Only, the deductions you paid in under CSRS will be used to pay both the amounts required for retirement deductions under CSRS Offset or FERS, as applicable to you, and also the Social Security taxes that you would have paid had you been in CSRS-Offset, FERS, or Social Security-Only.