depends on what your adding and at what EC it wont screw with it so you can add away as your drain to waste, say your res is 0.7ec and you have 5 liters left if you wanted to bump it up to 0.8 you would add 5 liters of 0.9ec feed water. Just keep the ratios the same.
Where one may want to swap out a reservoir is if its a recirculating one and depending on how long the last fill was.
Also to add as EC goes up in your feed water “typically” your pH will also drop but that all depends on the nutrient make up, but typically that is what happens.