We can observe all nows that exist/ed in our past light cone and know that laws are atemporal within those observations.>We don't know what's subject to change and we don't know if we can do things that can change the current behavior
We can know what is subject to temporal change and what is not by exposing the laws to temporality: observing the law in time as time acts (or does not act) upon it. If laws were subject to temporality they would vary from moment to moment.
The hypothetical possibility of future variance you postulate can't be caused by temporality itself, it would have to be caused by will (or something similar to will). A theory of occasionallism where cause and effect are consistant only because God/universe wills it to be, perhaps out of habit or a divine inertia, but that will, and the laws that will puts into effect, can change in the future.
C is 299792458 km/s only because God/universe wills it to be, but if God/universes will changed it could become any value.