I have it on very good authority that the PLA has stepped up recruiting efforts in Liaoning and Jilin provinces.
These provinces have large number of Chinese people who know Korean and who have knowledge of the border region of North Korea.
The Korean border is very porous in that area.
This may not be so ominous. The Korean peninsula has destabilized (well, it hasn’t been that stable in recent history actually) lately, and things heat up, our first priority would likely be to protect Seoul, while waiting for reinforcements to maneuver against the north. If the PLA helped out by striking from the North, I don’t think we would have a problem with it. If you recall the decades of problems Germany had when East Germany was unified due to the economic disparity, you can imagine the South wants know part of rebuilding the North and sustaining them for the next fifty years while they get on their feet, nor do we. China on the other hand, may be interested.