Yes, you can amend your return to claim your dependent and change your filing status if qualified to do so.
I received a reply from the IRS on my amended return stating:
“We could not change your filing status from single to head of household because the taxpayer identification number belonging to your qualifying dependent was assigned after the return due date for the tax period listed above”.
Is it worth appealing this decision?
You might want to plead your case based on this information from the IRS.
If you are hoping to receive a bigger refund and get child tax credit, etc. by amending and entering your child's SSN now, you will not be successful. The tax law does not permit you to add a child's SSN to get child-related credits if you filed without the SSN.