question about ACA estimated income issues

Does anyone know what happens if you overestimate your 2014 income when applying for the subsidy (tax credit)? For example, if you state an income level on your application and your premium with subsidy is based on that number, but at the end of 2014 when you calculate your tax return you find out that your income is less than what the original application amount was--what happens then?
Do you get the difference refunded in 2015?
I asked this question to a ACA phone rep (Calif) and she did not know. She said that if you underestimate your income when applying, that you will owe the difference when your 2014 return is reconciled, but had no information as to the result if you overestimated it--whether it would be refunded or just a loss.
Our tax returns are complicated and we never really know the true taxable income until the returns are prepared by our accountant so I am wondering about the consequences of an inaccurate estimate in the application process.