When you change flavors you don't have to use every last drop. I found with those tanks that doing that gave me a burnt flavor because the coil wouldn't get enough
juice on it. When mine gets to the point it needs filled, I add in what I want. Usually within a few puffs the new flavor has taken over. I have also went the route of dumping the little bit that is still in there, rinsing my tank and refilling

It's a personal choice but if you don't mind the mixing of flavor a bit, I say add it in!
