A lot of the BB are in the substrate, on the walls and on the decorations too, when you are dealing with really low bioloads- you usually can change out your filter media without issue, however, its always a good idea to monitor for spikes and make that extra water only changes if needed.
Also, you don't need to use charcoal and change your filter out all the time-its a personal choice-it won't hurt to leave the used up charcoal in the filter or you can cut a slit and dump it out saving the sponge part and giving it a rinse/swish a couple of times a month.