如何识别动态增加的数量的对象

Let's say I have a set of images and I'm going to use these images to identify people who are coming to the office. Then I build my own face recognition and I train the ANN to recognize faces. But a month later, there's a new employee, should I re-train the ANN (entirely)? Is there any 'dynamic technique', so I can just train the new one not the whole images?Let's say I have a set of images and I'm going