What if, with a PoD sometime around 1950, the geopolitical situation in East Asia had resulted in a sort of swap between North Korea and Taiwan, with the former being fully conquered by the US-backed southern regime while the latter is taken over by the PRC?
[note- an intentional, diplomatic "trade" isn't going to be made, a circumstantial, de facto "trade" could "just happen" to "fall in to place" however, with the right PoD and alterations]
In January 1950, Mao and the CCP Central Committee fast-track the planned invasion of Hainan by about a month from 5 March 1950 to 5 February 1950, despite continued mopping up operations against small KMT troop contingents in mountainous southwest China.
This earlier invasion is also luckier in terms of cooperating with Communist guerrilla forces on the island to outflank Nationalist defenses, possibly in part because the attack becomes earlier than the KMT ground commander, Xue Yue, and naval commanders expect. It succeeds in overwhelming the island, seizing key terrain and forcing evacuation of KMT remnants in 2 weeks rather than the historical 7 weeks. This successfully grabs Hainan in the ATL before OTL's Hainan campaign even began, and boosts Beijing's confidence and internal demand to finish off Taiwan during 1950, including pressure from Mao to make an attempt even before spring is over.
As a result, Beijing throws together an invasion force to launch an assault on Taiwan on May 15th, while masking Quemoy/Jinmen, rather than assaulting it head-on. Landings in a a couple locations fail, but landings elsewhere succeed, and are not thrown into the sea.
The Taiwan campaign is much longer and harder fought than the Hainan campaign OTL, or ATL. So more than 7 weeks certainly. The issue is in doubt for the first several weeks and the PRC lodgment only ensured by follow-on landings and reinforcements and some misfortunes on the KMT side.
The Truman administration, sticking with prior policy, despite some internecine debate and second-guessing by politicians, does not intervene in Taiwan in May and June 1950. However, during this time, the US still blocks the PRC takeover of China's UN seat, since KMT forces are still fielding major ground formations, conducting air sorties and bombardments in Taiwan and the near coast of the mainland, and Chiang is still doing radio and press communiques from urban centers he controls. Therefore the Soviet boycott of the UN continues.
However, North Korea attacks South Korea on June 25th, and the Truman Administration decides to draw a line there, and intervenes, gaining UN endorsement.
With a ground melee going on in Taiwan, the OTL "neutralization" formula and interposition of the fleet would be insufficient to stop the Taiwan fighting, so Washington DC needs to decide to intervene in a combined arms fashion, or not at all. With South Korea the priority, and closer to Japan anyway, intervention in Taiwan or the straits is skipped entirely. As it was, in the initial weeks in OTL, the 7th fleet interdiction was largely a bluff with few units on station, and plenty of work for 7th fleet units to do in Korean waters.
During the early weeks of Korean War fighting, Chiang's hopes for an American policy reversal and intervention rise. Even members of the administration, like the President and Secretary of State Acheson, who oppose a reversal of the non-intervention policy toward Taiwan policy find it convenient for Chiang to remain hopeful and thus prolong his resistance. The CIA provides covert assistance to Chiang, routed via the Philippines.
Beijing's leaders and planners are operationally focused on Taiwan and finishing the job there in the early weeks of July. In principle, American intervention in Korea is disturbing, but Pyongyang says it is doing great on its own, and that is what the maps show in July.
In August, as the Pusan perimeter consolidates, the situation gets more disturbing for Beijing.
The Communists have finally routed the Nationalists on Taiwan by about 30 August, and Chiang Kaishek reveals his retreat, from Manila, on September 4th.
However, the 10-week Taiwan campaign, and the relative complacency in July have delayed the formation of the Northeast Frontier Force by China by at least a month, and it is less well-staffed and reinforced in Manchuria at the outset.
After September 10th, and the Inchon landings that day, things rapidly get worse in North Korea.
US-UN-ROK forces advance into North Korea in late September and October while the Chinese rapidly redistribute forces northward to protect their northeast, move Nationalist PoWs and civilian PoWs to the mainland or prisons on Taiwan, and draw-down on Taiwan from peak combat strength on the island.
In October, Stalin presses Mao to intervene in Korea. Mao seriously considers it, but demands heavy Soviet support. That is not forthcoming. As in OTL, Lin Biao calls in sick and refuses to command. However, in this ATL, although confident in, and proud of their abilities, other senior commanders follow Lin Biao's lead and drag their feet, claiming lack of time to rest and refit sufficient veteran troops, or make operational or logistical plans in time to save the North Koreans. North Korea is snuffed out in November while this discussion continues without any major Chinese intervention, and Kim Il-Sung crosses the Yalu into exile.