In terms of your monthly payment, that part gets sorted at tax time. If you underestimated your income and should have been paying say, $50/month more, then they'll just assess it as owed when you file taxes. Same if you overestimate; you'll get a refund.
If you underestimate your income for the cost sharing however, it doesn't result in any claw back on things like lower OOP, deductables, copays, etc. The cost sharing subsidies for the silver plans are just there. If you severely underestimated, or do this several years in a row, they may require more documentation from you in some form for the basis of your next yearly assessment. I personally don't feel the need to test the cut offs on that one myself.