Contrary to "add on to", one antonym would be "take away from". This denotes the act of reducing or subtracting from something, as opposed to adding or enhancing it. Another antonym for "add on to" could be "simplify" or "streamline", where you are simplifying or removing extraneous components to make it more efficient. Other words that can serve as antonyms for "add on to" include "cut back", "scale down", "diminish", and "trim down", among others. Whatever the antonym used, it denotes the act of reducing and simplifying rather than adding and expanding.