It might change but it is not very likely I think.
You can experiment with energy. If you do the right experiments you will find out that you are not able to create energy. Not even a powerplant creates energy out of nothing. You can only create it out of mass.
This tells us that E/m = constant. E is the total amount of energy in the universe, and m the total amount of mass in the universe.
Einstein showed that the constant is c^2.
If c would not be constant we would be able to create energy out of nothing. And we can't.
Indirect emperical evidence that c is constant. Indirect, but still found by experimenting.