Up or down

When and how did up and down come to an agreement?

The phrases "something's going down" and "something's up" now mean the same thing.
Like, you and a buddy are wondering what's wrong with a friend. If something is bothering him or her. Someone says, "something's definitely up." The other nods in agreement. "Yup. something's going down."

And then someone asks me to a movie, or coffee, or something. I can say, "I'm up for that" or "I'm down for that." Either way, I mean the same thing.

How do these things happen? I'd love to be the person to begin a similar trend. To be the person who says, Ok, now I'm going to start using "down" as if it's "up." Can I just make up my own? If you hear me saying, "Something's around..." that means "Something's up." You down with that?