Once the IRS has accepted Form 4868 there is no way of changing it. It is not like a tax return where if you make a mistake you can amend it. However, the important thing is that you pay as close to the exact amount of taxes that you owe by April 18, 2017. With that in mind, I would create a married filing jointly tax return with all of the information that you have today in order to get as accurate a picture on the amount that you owe, and make sure you send that check in for that amount before the April 18, 2017 deadline.
