Not even then.
Agreed. Occupation of the US is pretty much impossible. They might be able to occupy a couple of cities for a while, even storm the white house and take some leaders as hostages, but even a modern army like the US army cant occupy a country like the US. Just look at the difficulties with a geographically comparably simple country like Iraq, or the slightly more difficult Afghanistan. A permanent occupation would simply not be possible. Maybe, maybe if they managed to conquer all the nuclear silos and replace the fried electronics in them, then retargeting them at US cities. Maybe then. But then resistance would be very different from the original movie, Red Dawn and the consequences might be much more difficult. Of course the rest of the world would simply answer by nuking the shit out of NK leaving the occupiers in the US without leadership and a home, causing the operation to fall apart.
I think the the movie is going to tank. Reda Dawn worked because it was the right movie for the time with the cold war at its high and everyone in the US just ready to "kick the shit out of them red basterds".
Today NK is just an oddity, an anachronism, a curiosity and occasional hate target, but not a real big threat and certainly not on peoples minds all the time, especially with the economy like it is (they have other problems).
China could be a bigger threat if they had any real interest in starting a war with the US. If China starts trading oil in yuan instead of USD, the US would have more economic reason to start a war with China, than the other way round...
So again, no credible scenario there.