UPDATE: After a discussion with a Covered CA representative today (Darlene - very helpful!), I feel like I have a clearer understanding. While it's true that a mid-December Roth conversion will not affect your Covered CA premiums, because the additional income occurred and was reported after the final premium for the year, that's not the end of the story.
Basically, you'll need to settle up with the IRS at tax time. Covered CA will report the tax credit benefits you received in the form of reduced premiums (I think form 1095-A), and the IRS will require any adjustments based on your total income for the year and the total tax credits that are allowed for that income level. So, even though you won't pay any higher premiums, you could owe more taxes. Depending on your situation and the amount of money converted, you might end up having to pay back *all* of the financial assistance (i.e., premium reductions) you received during the year.
