A designation is part of a statute, which is a law. Ex Post Facto covers changes in a law as well as new legislation. When you change a law, it effectively becomes a new law.
So it is illegal to call Negroes "African-American" unless they were born after that "designation" was added to the law? Ex post facto applies to making something criminal (punishable) and applying it to actions which occurred before the law as passed. Unless there is punishment, there can be no ex post facto.