If I upload a new theme, will it mess up the Posts, threads, profiles, reputation or anything like that? Will it just change the layout/theme?
Uploading a theme has no affect on any database data - it only affects the frontend, not the backend.
It's only HTML and CSS.
You will lose any plugin code in the templates though as the theme will import clean new templates, unless you edit the settings of the new theme to sue a template set that already contains these changes.