For a game I recently submitted, I'd like to make a slight change to the overview description and changelog. Given that mods are the ones supposed to make updates to game submissions, am I allowed to do this? Would it affect the game's searchability or anything?
It'll be fine as long as you're actively helping, not screwing with the community. A common pitfall is the case where the dev posted v0.1 here, then when v0.2 comes out he's hoping to keep it Patreon only for a month, but of course it's almost immediately leaked. In a rage he comes back here, edits the links to put back the 0.1v, and then has the :surprisedpicachu: face when he loses control over the thread.
If you're just adding more screenshots, tweaking the text, adding in extras, etc, you'll be fine.