Totally irrelevant to the topic, but their robot design looks like R.O.B.
On topic, they don't actually have any physical duties by the sounds of it, they just perform monitoring services that humans may not be adequately equipped to do, e.g. the aforementioned mood-change detection. So it's not that radical of a change, it can be likened to installing new equipment like cameras in the prison. Guards still have to perform their guard duties, but they now have more equipment to help them perform it better (at least, that's hoped).
The ethics behind it are a little more intriguing. The actual installation of the robots in the prison is hardly interesting. However, they give indication that they don't plan on the robots taking over the guards' jobs, which means they won't likely get in a situation where they actually need to interact with a prisoner, peacefully or defensively. However, it's interesting to see if they will actually follow through with that statement, or if demand for putting humans in less risk results in something similar to a UAV: guard robots controlled by humans in a secure facility. This results in the question of whether guard safety trumps guard efficiency (robots are limited in some ways compared to humans), and a myriad of other possible dilemmas, most related to human interaction with prisoners being replaced with an iron facade that robots provide. And then there's the question of whether the anonymity of remote controlled robots could result in further abuse.
It's an interesting project by the Koreans, and I'm interested to see how they go about with it.