The Supreme Court can not "change" the constitution. They are sworn to uphold and defend the constitution as written. However in recent times they have taken to reading the constitution and declaring that it means something completely different than what it actually says, or was intended by the writers. If Congress tried to pass a law forbidding a cross on public property (or, for that matter, mandating one to exist) THAT would be unconstitutional. The first ammendment is only a restraint on Congress.