Antonyms for the word "more preexisting" could include "less existing," "new," "recent," "fresh," or "novel." These terms convey the opposite of "more preexisting," implying something that is not already established, established for a shorter period, or introduced for the first time. When describing something that is the opposite of "more preexisting," one could use these antonyms to describe something that is unique, innovative, or original. Alternatively, if one wishes to convey that something is less pre-existing, the described object could be considered less commonplace, less old, or less established.