In 779, the Uyghur khagan Bogu wanted to invade China, but was opposed by his general Tun Bagha Tarqan, who eventually killed and overthrew him. If Tun Bagha Tarqan never challenges the Khagan or Bogu Qaghan manages to defeat him, could the Uyghurs have lead a successful invasion of China? If not, are there any better times that the Uyghurs could have conquered much of China, like the Mongols and Manchu did?
One of the reasons why I chose this POD rather than any earlier ones is that the Uyghurs had officially converted to Manichaeism by this point, which makes things interesting...