The only time you should upgrade a body is when the body you have can no longer produce the images you want because of its limitations (not user limitations...). So if you have a D200 but all of a sudden decide you have to shoot a lot of low light, interior shots, then it will be time to up grade.
I shoot professionally using a D2Xs and D200, but I shoot in a country where there is an abundance of light so high ISO performance is not really an issue. These 2 cameras are fine for me at the moment. However, I am moving back to the UK shortly and think that maybe it will be time to upgrade as low light work becomes more common place.