Knowledge memory doesn't really work that way, it is more like that it is constantly fading unless re-imprinted by use and learning new things is just imprinting new knowledge on top. The new knowledge will form connections with the old knowledge which will help keep some of it from fading, but not all.

Another way of looking at what you said is that the practicing the new knowledge takes the place of practicing the old knowledge. So it isn't the knowledge that is replaced, but the learning (imprinting).