If I understand correctly, most people will not be affected negatively by this, as the tax tables will be changed along with the withholding tables... so you should get $400 more throughout the year, and owe $400 less at tax time. The only way you could owe more (or get less of a refund) is if you accidentally get to much throughout the year (aka, maybe you get $500 throughout the year, and owe $400 less). So, you are just paying back money you shouldn't have gotten.