I have a client that had an ITIN, but has now received their SSN. Can I amend their prior year returns to claim the EITC that they would have otherwise qualified for?
No, a prior year return that was filed with an ITIN cannot be amended with a newly issued SSN to claim EITC. This rule was set down by the PATH Act of 2015.
"The Protecting Americans from Tax Hikes or PATH Act made several changes to the tax law, including preventing retroactive claims of the EITC by amending a return or filing an original return for any earlier year in which the individual or anyone listed on the return did not have a SSN valid for employment. You can't claim EITC unless the Social Security number for you, your spouse (if married filing a joint return) or a qualifying child is issued before the due date of the return including any valid extensions."
Further information regarding the rules for claiming EITC can be found in Publication 596.