If every box on your W2 remained the same except for box 12a, then there would be no reason to amend your tax return. You should assume all the other numbers stayed the same. it appears your employer reported the wrong amount of 401(k) contributions for you, however, since they didn't correct box 1, Gross Wages, then that figure was reported correctly originally.
You would think box 1 would change also, but it's very possible that only the 401(k) amounts were incorrectly reported originally. You can check with your employer to be certain, if you are unsure.
If all other boxes remained the same, there is nothing for you to correct or amend on your federal return. Be sure to check your state wages, state withholding as well as federal withholding.
