I might be wrong, but with DWC I've been reading that swapping out the entire tank once per week is strongly suggested. It makes sense, in that it makes sure you aren't building up high concentrations of nutes that the plant is not using, and more importantly, make sure it has enough of the "least available" nutrient that it needs.
The thing is that I have yet to do that. And I have yet to see a failed plant. Granted, I only have two finished grows so far, but the first one was a 1/4 pound yielding monster.
To keep up with the schedule for a young plant in DWC means I would have to add nutrients directly to the tank (which is also frowned upon) or replace the solution with the new mix. I dunno. Maybe I am overthinking this.