Opposite words with the same meaning

Is there a term for the phenomenon when you can replace one word in a sentence with a typically opposing meaning word and maintain the meaning of the sentence?

Examples:

I'm down for that!
I'm up for that!

or

Slim chance.
Fat chance.

Admittedly, "fat chance" probably originated as a sarcastic derivation of "slim chance", but in practice, they convey the same thing.

This is slightly related to this question, but that one's asking about a specific case (which may or may not fall into the phenomenon I'm asking about). I'm just looking to see if there is an established term I can use to search for more examples.

Solution 1:

Words that have opposite meanings depending on context are called contranyms; Janus words, from the Roman god of doors between beginnings and ends. It's not exactly what you're looking for, but the term might get you on the right path.

But I'd guess most examples include neologisms — new colloquialisms we may use but might not be widely "accepted" yet, like "sick" meaning an awesome new DLC for a staid game, where the word ("sick") also retains it's everyday meaning of "not well". The neologism ("sick" with a positive connotation) is then combined with an antonym for the original word's meaning (antonym of "sick", like "healthy"), to produce the effect the OP is asking for: neologism[word] + antonym[word].