It is not a good idea to convert true brackish species, as it will result in stress in the long run, if not causing problems outright. However, temporary brackish species, such as Scatophagus and Monodactylus are actually marine fishes and should be kept as such in the long term. It is a good idea to have an accurate means of measuring the salinity/specific gravity when dealing with brackish species, especially if you are trying to bring their water to marine levels. They are not safe with reefs or macroalgae, BTW.