USA citizens needing to change from L Chinese visa to X Chinese visa

Hey! We are USA citizens. My son is studying in China but he hold a Chinese L visa expedited on May 2023. Now the government is asking us to change the visa category from a L visa to X visa. The local government said that they can’t do it from here that we need to travel to Korea.. I don’t understand the reason but I need to know if as Americans we are able to get an X Chinese visa for my son from South Korea and if yes; the how long it takes! Thank you!