With a change in the speed of light, consequent changes would appear to all related physical properties of the universe. We would move relatively slower and time would dilate to match our reduced pace. Consequently, everything would appear as it does now. Maybe some events would have been slightly mis-timed such as the attack on Pearl Harbor and the launching of the A1 rocket by Nazis Germany so that history would have progressively changed. So, where would we be now? Maybe Apple wouldn't have dominated the techno-sphere and Microsoft wouldn't look so like a stodgy seventy-year-old.