Do you mean Case Law as in Plyler v. Doe where the Court ruled that the children of illegal aliens could not be denied protections granted under the 14th Amendment.
The way the law would work is that what was in force at that relevant time would be the law one would be tried under.
So, with Plyler v. Doe, that came 21 years after his birth, so it wouldn’t apply to his status at birth.
For those interested:
http://en.wikipedia.org/wiki/Plyler_v._Doe