Source: Blank, A. 1999. Why Do New Meanings Occur? A Cognitive Typology of the Motivation for Lexical Semantic Change. In Historical Semantics and Cognition. Edited by P. Blank Koch, 61-90. Berlin/New York: Mouton de Gruyter.
Antiphrasis: Change based on a contrastive aspect of the concepts, e.g., perfect lady in the sense of "prostitute".
Auto-antonymy: Change of a word's sense and concept to the complementary opposite, e.g., bad in the slang sense of "good".
In both, the original word adopts the (polar) opposite of its original meaning. So how do these 2 semantic shifts differ?
This deleted Reddit comment doesn't expound the distinctions enough, and I still can't distinguish.
“Perfect lady” and “prostitute” are, even if they could be considered opposite, are not antonymous in the same way that “good” and “bad” are. A lot of times we use antiphrases when we are speaking sarcastically, but you would not say that someone who exclaims “wicked!” is being sarcastic or is intentionally saying something that they mean the opposite of.