Well, they didn't do it in one day. There was a post a few days ago on a documentary about Tibet. It's from CPC-affiliated media, but it uses Western sources. Someone also commented a link to a CPC article talking about how they did it.

The PLA basically entered by signing an agreement with the theocratic government and they started growing crops and establishing schools, until reactionaries forces tried to drive out the PLA which invited support from the serfs to change the regime.