As UN forces advanced up the Korean Peninsula, and amid increasing warnings that China may intervene, (along with Chinese statements themselves) suppose MacArthur accepts the possibility and need to prepare, but is still committed to an advance to the Yalu. Much better PR for him that way--can't have people saying he was scared of the commies! So, the advance to the Yalu itself past a certain point is much lighter, with a fall-back line set up with defensive preparations in the event the PRC enters the war directly.
What else would have been done to guard against the possibility of holding Korea against the potential size of a Chinese intervention? Would this precautionary behavior be enough to satisfy Mao and his inner circle that the UN advance would not proceed into China itself? More skeptically, would the risk of attacking into prepared defenses even be a deterrent?
The Korean War was costly enough for the PRC IOTL for it to have been a bit shaken, presumably ITTL the situation would be worse unless there's an earlier cease fire.