Change those constants, and you change the way the Universe works. Including unimportant things like chemistry, and electrical conductivity. And outside of a surprisingly narrow range. . . life is impossible. . .
Some good points. But when we're talking about a possible change in the speed of light, perhaps it happened before life existed on Earth, eh?
You keep asking all these perhaps and might it be possible questions. Can’t you think in principles. The answer to all of these hypothetical questions is the same. Prove it. Existence exists. Truths are absolutes.
At one time man believed, wrongly, that the Earth was flat. Within the context of the knowledge available to man at the time there was some evidence but not enough evidence to conclude the Earth was flat. When man advanced a little and was able to measure and prove that the Earth was actually round, this new knowledge did not overturn an absolute truth but a false conclusion. But now we have proven with mathematics that the Earth is round and we have also flown out into space and proved it by perception. In the same manner man has proven the constancy of C. By the nature of the universe the fact that the Earth is round is an absolute. We are not at some point in the future going to make some discovery that overturns that truth and makes the Earth flat again.
It is the same with light. You could say that monkeys with tennis rackets are out there in space doing backhands to speed up photons and then jellyfish are lining up to slow the photons back down. The answer to all such suggestions is the same. Prove it.