If you didn’t amend your 2017 tax return, you can use the IRS DRT. Once your FAFSA has processed, you can go back into it…and link to the DRT.
It won’t create any changes UNLESS the info from the tax return is different than what you entered. The info from the DRT or a tax transcript would be the real deal.
So…yes, you can link to the DRT.
Submitting info after your initial submission will NOT hurt you. How would that be the case?
Did you apply early decision? Does this college also require the CSS Profile?