This market will use the population contained within the borders of the countries even if they change, and even if the form of government changes or there is large migration. The population is the number of people living within the political borders at the time, not necessarily just citizens of that country.
2021 status:
South Korea; 51.7m
North Korea: 25.97m
1,000
1.00OWID forecasts say no. Even comparing the 'high' scenario for North Korea against the 'low' scenario for South Korea doesn't get you there by 2038. Even fairly dramatic changes in fertility outside that range are unlikely to show up in only 15 years.
This would require something fairly far outside the ordinary. Significant changes in borders, massive casualties from war, mass migration, etc. I haven't checked, but I suspect we haven't seen events of the magnitude required since WWII.
@StrayClimb Disease having a large differential effect and hitting the wealthier country that much harder would be pretty wild. I'm not sure I know of any good historical analogy, even including massive pandemics like the Spanish Flu or the Black Death. But yes, it's a theoretical candidate for a mass casualty event.
@EvanDaniel yep I'm just trying to create some shorter term markets for the Korean peninsula. It's surprising to me how little attention is paid to it.