I'm doing my first legit nute change right now. I pulled the water out of the system. And ran 7 gallons through the plants as I would if I top watered to wash out the roots, so less contamination of the previous nutes into the new nutes. Ive been talking about how finicky the Silica is im about to use. I need water a clean as I can get it. Also this watering will help keep the roots as the RO water refills the system, which can take a while. The Silica HAS to be the first nutrient added.
Different strains have different tolerances to this period of being out of water. Some need to be top watered consistanly until the roots are comfortably supported in water.