I've come across both sentences and was wondering if there is any difference in their meaning. Here are the sentences where I met them:
To be down:
Give me a call if you're down
To be in:
Who's in for the show tonight?
Can one switch them without any impact as in
if you're in and
who's down for ?
Edit: a new challenger joins the list,
to be up to:
Tell me what you're up to
Also, I would point out that I've read/heard that in America and English is my second language.
Now the main question becomes: Can we interchangeably use "to be in for", "to be down with" and "to be up to" ? Can we also switch the extra words such as "to be in with", "to be down to", "to be up for" ? If you have any concrete example where one or another could be more appropriate/meaningful, please let us know!