How does RSS feed pick up posts? Would old posts be safe?

I successfully installed a plugin that excludes certain categories from my feed. But I would like these old posts to become part of the over all Category setup, not just hang around in that special Excluded category. What I can’t figure out (and I’ve searched! here and online!) is what determines how the feed picks things up, and when.

If I have the post in Excluded Category, and then, a week later, change it to another Category that would normally be included in the feed, will this post then activate and show up? Or, because it is a week old, the feed won’t grab it, since I imagine it grabs posts that are dated SINCE the last time it pinged?

Thanks for any input. I’m sure this falls in the category of “things so simple no one tells you…” but I obviously need to be told!