>>508892511no, idiot. it won't be applied retroactively.
also, when you're born, you don't get a SSN (which amounts to "true" proof of citizenship) right away.
your parents have to bring your birth certificate to the SSN office and they give you a SSN.
birthright citizenship going away just means your parents have to bring one more piece of paper to the SS office.
namely their (or their spouses) birth certificate showing at least one of them was born here.
it really is that simple.