1) First of all, what is the term for this?
2) Does it fall somewhere under the umbrella of contronyms/auto-antonyms, or is it something entirely different?
3) Is one way more correct than the other?
I'm not in a position to answer your questions, but I'll mention that while someone else made the observation that fat chance and slim chance both mean the same thing, George Carlin pointed out that what's coming off? is identical to what's going on?, and what's coming up? to what's going down?
Another favorite Carlinism of mine was his comment about flight attendants instructing him to "get on the plane" to which he'd reply "Fuck you! Let the daredevils get ON the plane ... I'm getting IN the plane."
Responding to alexkelly's original question, I sense a slight difference in meaning. "Are you up for it?" would imply "Are you available or interested?" and "Are you down for it?" would imply "Have you already signed up or RSVPed or however committed?"
But like Ron, I'm not sure there's a specific term that applies to this apparently contradictory use of "down" and "up." There's so many examples of words being turned around like this (for example, "bad" meaning "good") that it seems there should be a term for it. However, this might just be another example of language evolving as dictated by common use.
I can understand how "are you down for this?" would sound to some people like "have you signed up for it?" (ie "is your name down on the list for it?"). But when I hear it, I think of '60s and '70s hippie-speak (or maybe it was a little after the hippies) meaning "are you willing to do this?", "can you commit to this?" or even "will you follow through on this?"—in other words very close to "are you up for this?".
I have not heard "down for it"; rather "down with it." I don't believe it was in use back in my hippie days. I think it's more connected to the rap/hip-hop era.
I don't know if there's a term for these seeming opposites.
This reminds me, and I may have asked before, please forgive me, but my wife, among others, tends to say "I'm fine with that" where I would say "That's fine with me." I think the former is part of the same package as "down with it," something my wife picked up from the kids while working as a school secretary. Anybody know more than I do about when or where this change may have come about?
Apologies if this constitutes a hijacking.