As others have said, as long as the D200 (or any photo equipment) does what you need it to do there is no reason to upgrade. When your photographic journey takes you to a place where the equipment is no longer able to capture the images you want, or equipment limitations are keeping you from moving into a new area of photography that you are interested in, then it is time to upgrade. Your plan to work on your own photographic skill and technique is a good one.
The D200 is still my primary camera and it is as capable today as the day I bought it. I would like a camera with more dynamic range and better high-ISO noise performance, but I do not really *need* these performance features right now. Eventually I will upgrade the camera body but, for now, I am still happy with the performance of the D200.