As others have said, the backstory is what you use to write the world. It's why this city is ruled by a council of elders while that one has a king and the other one has a 30' city wall, why elves are rare and why left-handed people speak a different language. And so on.

Change the backstory, and you need to change the world to match it. Or rather, change the world, and you need to change your backstory to match it. Otherwise your world becomes incoherent.

That might or might not matter to your players, depending how much attention they're paying and how much they expect to be able to use their natural intuition about the world. But it'll surely matter to you.