antecedent vs pre-existing

antecedent is a noun but pre-existing is not a noun.

Word	Noun	Adjective	Verb	Adverb
antecedent	Yes	Yes	No	No
pre-existing	No	Yes	No	No

antecedent (noun)	pre-existing (noun)
the referent of an anaphor; a phrase or clause that is referred to by an anaphoric pronoun
a preceding occurrence or cause or event
someone from whom you are descended (but usually more remote than a grandparent)
anything that precedes something similar in time

antecedent (adjective)	pre-existing (adjective)
preceding in time or order	existing previously or before something