mean the same thing but have different routes. i just have found an interesting explanation: " ...So there's the possible origin of "I'm up for that." It was first "I can get up for that". "I can gear up for that." - Got shortened to 'I'm up for that!" "I'm down with that" might have come from "Whose name can I write down for volunteering this Saturday?" "You can put my name down". " besides i guess "up" is more about some action (ready to go) and "down" is more about agreement with the current theme for example

They're absolutely interchangeable in meaning. However, I'd say that "down for" is less formal, almost slang, whereas "up for" is more old-fashioned. I can imagine an elderly person saying "Are you up for_____?" but not "Are you down for______?" Hope that helps.

They mean the same thing but contrary to what was stated here, 'down for' isn't new. I remember hearing it back in the 1970s.

This link says they mean the same. https://www.quora.com/Why-do-Im-up-for-it-and-Im-down-for-it-mean-the-same-thing However, I have never heard "I'm down for it"; I have heard "I'm up for it". My advice is to stay with "up for it". Perhaps "down for it" is British.