Switching from bromine does require a complete water replacement. Trying a baqua conversion would simply have no effect (other than to waste chlorine). Bromine does not need to be expensive. Once you have an appropriate bromide level you can simply use chlorine to reactivate it back to bromine. You will use somewhat more chlorine than you would without the bromine, but you don't need to buy any expensive chemicals other than recharging the bromide bank once or twice a season.