This was surprising for me.
"I am up for it" = "I am down for it!"
Let's build a case.
We are a group of 5 people. Two want to watch 'Titanic', I want to watch 'Avengers'. Now, I want remaining two to be with me! Shall I use 'down' for 'Titanic' to ask them?
Hey, you both... are you 'down' for Titanic?
I'm actually giving them a hint by having 'down' and 'Titanic' together.
On the other hand,
Hey, you both... are you 'up' for Avengers?
Here, should they consider 'up' and 'Avengers' and say, 'yes'?
Furthermore, do these sentences have any other impact semantically? Am I going to 'lose' those two votes if say like this -
Hey, you both... are you 'down' for Avengers?
Hey, you both... are you 'up' for Titanic?