Of the phrases “down with it” and “up for it”?

Depends.

Are you down with it?

Or are you up for it?

I’m down with it if you want to get breakfast after we check traps in the morning. If you’re up for it?

Could go the other way too.

I’m up for it if you want to get breakfast in the morning after we check traps if you’re down with it?

So they are synonyms for “I’m good”?

No!
Absolutely not!
That could mean the exact OPPOSITE, given the right vocal inflection and context.

" " You want another beer?"
" I'm good"

Opposite of;

" I'm down with that homie"
Or
" Yeah, I'm up for dat"

So are down and up the same or is down up as up is down?

Both are slang, with similar but not the same meaning.
Down with it=are you okay with it. Could be used in the context of asking someone if they want to go somewhere, if they're okay with you doing something, if they agree with something, etc. Indicates whether or not someone is okay with some action or idea. Doesn't necessarily ask whether or not they themselves want to participate in an activity, just asked about their approval of the activity.

Up for it=a more specific version, typically only used to ask about someone's willingness to participate in a particular activity.


Example....My wife doesn't want to join me in going out to call raccoons, but she doesn't care if I go, so long as she doesn't already have something going on. So I'd say....."hey beautiful bride, I'd like to go out calling calling, if you're down with that." I'm not asking if she wants to come with, only if she cares if I go or not.

I know my oldest son would love to go, though. So I'd ask him.."hey, i'm going out calling raccoons this afternoon, you can come if you're up for it." In this case, I'm specifically asking if he would like to join me.

What was hot became cool and then sweet but are now sick, and then up was down, and fat chick (bad) became phat chick (good), and good became bad, and whatever the next generation wants to make it to distinguish themselves from the previous generation that isn't hot or cool or sweet or sick anymore.

Sounds like changing speech is a natural progression.

Yep

To me:
“down with it” means I agree with whatever you just said. Mere agreement to a proposal.

“up for it” means I agree with whatever you proposed to do. Actually taking action.

More like for "I'm Willing".

Neither is "proper". People who use those phrases are trying to sound more "cool" than they actually are.
Just say exactly what you mean instead. Speak English like Jesus did in the Bible...