Those governments also fell internally, meaning the people were pushing for a change and were somewhat ready to make it happen. Invading North Korea would be pretty different.
One big unknown (at least to me) is the level of indoctrination. Related to the above, a lot of people in Eastern Europe were fed up with their government, didn't believe anything they said, and were ready to build something better. It seems like a lot more North Koreans might buy into their government's views, in which case they might flee the "evil imperialist invaders." Heck, even without indoctrination, a reasonable person might not want to be near the fighting.
25 million refugees is obviously a bit much. The entire country won't pick up and move. But it would probably be many millions.
[1]https://en.wikipedia.org/wiki/Refugees_of_Iraq#Iraq_War_.282...
Not Europe, no - because most of them didn't have the legal status to gain any european citizenship. But million jews rushed into Israel, with a total population of 8 million, which greatly affected the country.
Given that South Korea officially 'wants' unification, they won't have any other choice but to grant citizen status to everyone - and I don't doubt that a LOT of north koreans will jump on this opportunity right away.
Of course they'll have the choice not to immediately grant citizenship to 25 million!
Those were largely peaceful changes. Neither Germany nor the Soviet Union got nuked nor have millions of people die in a war, as is very likely to happen if war breaks out with North Korea. If they had, then yes, there would have been probably millions of refugees trying to run for their lives. Take a look at any other major war and you'll see that this is in fact what happens.
(Not trying to be snarky, genuinely curious, because every time this subject comes up, everyone starts throwing around the "China doesn't want 25M refugees overnight" argument, so I assume there's some background I'm missing.)