did you repay the sign-in bonus in the same year you got it, then the w-2c should show wages and other items net of the repayment so box 1 would change and you would need to file an amended return.
if the repayment was in a subsequent year different rules apply. the W-2C may only be to correct social security and medicare wages and withholding boxes 3 through 6 and no amended return needs to be filed for these changes
rules for subsequent year repayment
if the repayment was more than $3,000 (if less you can do nothing) you can take a deduction on schedule A line 16 describe it as IRC 1341 repayment OR
recompute the taxes in the year you received the payment by excluding the gross repayment. The decrease in taxes is taken as a credit on schedule 3 line 13b
