That's kind of a tricky question.
The Constitution specifically prohibits ex post facto law, that is, laws that retroactively alter the definition of a crime or punish conduct that was legal when committed.
No Bill of Attainder or ex post facto Law shall be passed.
US Constitution: Article I Section 9 ("Limits on Congress")
Ex post facto law, law that retroactively makes criminal conduct that was not criminal when performed, increases the punishment for crimes already committed, or changes the rules of procedure in force at the time an alleged crime was committed in a way substantially disadvantageous to the accused.
The Constitution of the United States forbids Congress and the states to pass any ex post facto law. In 1798 it was determined that this prohibition applies only to criminal laws and is not a general restriction on retroactive legislation. Implicit in the prohibition is the notion that individuals can be punished only in accordance with standards of conduct that they might have ascertained before acting.
Encyclopedia Brittanica: Ex Post Facto Law
Since there would be a significant new sanction for people previously granted the right of birthright citizenship, and would, potentially, make their presence here illegal, it is highly likely that any attempt to make it retroactive would fail, Constitutionally.
However, we are talking about an action, removing birthright citizenship which is specifically defined and granted in the Constitution, by executive fiat. If we're going to delve into the fantasy world where the Constitution and its rules do not apply, then I'm not sure why this provision would be binding when the core, fundamental one at issue is not.