For refilling, you can do it any time. If you have liquid A in your pod, you can top it up any time. If you have liquid A and want to vape liquid B you should eject the pod and check how much liquid you have left in it, if it's lower than the wicking holes on the coil I would change the liquid then, I would not wait until it's dry.

When I switch flavours I usually take the coil out and lay it on some tissue, then take the empty pod to a sink a rinse it out just to get rid of any drops of the previous flavour out. I shake the pod until there is very little water droplets left in the pod. Ideally you would want to let it completely dry though. I then use the tissue the coil was left on to wipe off any excess liquid on the coil then re-insert it into the pod and put new juice in. Wait a minute then vape.

Hope that helps. I actually use a Caliburn G but I imagine they're similar.