Well, North Korea is now the world's sole nuclear-armed nation, so that'll go about as well as you'd expect. A not-inconsiderable chance of Tokyo receiving a bucket of instant sunshine in short order, particularly if they do something stupid once they realise they've lost their main colonial holding (stupidity seems distressingly likely under the Taisei Yokusankai, which probably makes the Kims look level-headed and reasonable). Expect the Kwantung Army to get some unpleasant surprises in its backyard.
The Kim-Stalin interactions will be interesting, as will those between Kim Jong-Un and the young Kim Il-Sung (who is, after all, still Eternal President; there's a fun question of order of precedence for you).
Assuming the Norks don't try anything with South Korea (on the one hand, they've got a use-it-or-lose-it nuclear advantage; on the other, between the South, the uptime Americans, and downtime Japan and the US, it'd be suicide in the medium-term [1]), there's going to be an interesting exchange between the uptime and downtime Americans. US Forces Korea doesn't have nukes based in the Peninsula, but there's probably sufficient technical knowledge between USFK and the South to give the Manhattan Project a kick in the pants.
The real geopolitical question mark, beyond the consequences Japan going ballistic over the loss of the Peninsula, is going to be how this squares with the Neutrality Acts and the general lack of appetite for war. Assuming USFK recognises the downtime US government as the lawful source of authority, their hands are probably tied unless the North does aforementioned Very Silly Things.
[1] Open question of whether the North, who by all accounts aren't doing well with the closure of black market food sources IOTL 2020-21, bargain a truce with the South to focus on feeding themselves. If they were too clever by half, they could still maintain their long-term security by putting the rest of the world in the position it had been subject to uptime, re: "be good and don't get the Bomb, or we'll use ours on you".