Yes, it's still the same country. And nobody's being replaced if the Japanese are reproducing at lower than replacement rate.

If you replace a country’s culture, it’s still the same country?

Japan's culture has arguably been almost completely replaced several times over. Is it still Japan?

Change is the only constant in nature

That's just called changing culture. This happens at each generation at least.

yes. A country is a series of lines on a map.

also did you mean to imply that culture is essentially tied to ethnicity? because it isn't.

Well sure, the name of the continent is still "America", but the actual country, the United States of America, did not exist before European colonists.

So no, it's not the same country now.

The name is irrelevant. It’s certainly not the set of cultures it was before that event happened, no.