I am up for it 同意做某事

up for it

in British English

informal

keen or willing to try something out or make a good effort

it's a big challenge and I'm up for it

down for it/ up for it

They don't mean the same thing, even though they are often used interchangeably.

The origin of being "down for it" is probably (hard to get any hard evidence for this, but based on common usage) from "written down" in the sense of inscribed or registered. To be "down for" means you have made a written or other strong commitment to doing it.

To be "up for it" means you are willing or enthusiastic - probably (again I'm inferring from usage - and guessing a little) from the practice of standing up or coming forward when volunteering, as a means of showing willingness.

There are, therefore, often situations where they overlap, and I notice "down for" being used more often for passive experience and "up for" more often for active ones.

So I'm down for some massage but up for