It depends upon the tanks you are using. It also depends upon the flavor of the juices.
If using a clearomizer tank and you plan on changing to a similar flavor, you can just add the new flavored
juice without cleaning the tank. For instance, changing from a similar fruit flavor to another fruit flavor. But if switching from a coffee or tobacco flavor to something entirely different like a fruit flavor, you'd need to clean the tank first. Just rinsing it under a stream of water should be adequate. Possibly dry burning the heating coil a couple of times for a couple of seconds each.
If using a cartomizer tank, you would likely need to remove the cartomizer. A cartomizer has poly filler which retains flavor. Start a new flavor with an entirely fresh cartomizer (pre-filling it first, of course).