If your Gross Income from the Original W2 changed, so should the FICA (Social Security and Medicare). Basically it does not sound correct because your W-2C should reflect a change at least in Boxes 1-6. When you reimbursed your employer previously paid wages, your entire numbers changed because each number in Boxes 2,4 & 6, are a percentage of boxes 1,3 & 5. So your entire W-2C should have changed due to this reimbursement.
