I'm assuming you have a drip system? Roots will follow the path of the water and eventually end up in the bottom of the bucket. It is important to add an airstone to get oxygen to the roots that are sitting in the nutes. Plan on usiing your nute schedule and topoff with ph'd water till you exchange your nutes. In veg, I don't change my water but once every two weeks, then the second change is a transition solution, then finally to the flower nutes. When I'm in flower, I change my water every week, top off with water till the change.