The POD is in the Revolt of the Three Feudatories, where the Eight Banners failed fairly miserable. Instead of withdrawing them, the emperor decides to plunge all of them in, certain that they can't lose to the Han.
Well, they do lose to the Han, and the Banners are all wiped out. Frustrated, the emperor sends in the Green Standard Army as IOTL...
And it works!
Impressed, the emperor finally gets rid of the Banners and creates an army for all Chinese people who enlist.
By the way, this emperor is the Kangxi Emperor, the dude who continued Sinification with the Kangxi Dictionary.