Enclose a check for the difference between the original amount due on the erroneous 1040EZ and the corrected 1040X.u00a0 That should take care of the issue unless the difference is substantial enough to have interest calculated on the underpayment. If the difference is only a few hundred dollars of extra tax due then I wouldn't be concerned on calculating the interest. I would just wait for an IRS letter that assesses for interest due.