"Am one's case" is not a common phrase, and it doesn't have any widely recognized antonyms. However, we can explore some antonyms for the individual words used in the phrase.
"Am" can be the opposite of the verb "was," which represents past tense. "One's" is derived from "one," which is often used as a singular pronoun. "Case" can be the opposite of "benefit" or "help." Therefore, some antonyms for "am ones case" could be "was someone else's problem," "everyone's," or "hindrance." These words connote a different meaning from the original phrase and are not direct antonyms.