Sure. First off, what do you want it changed to? (as the above will remove the lock icon)
To switch it, you simply go to the font-awesome website ( http://fortawesome.github.io/Font-Awesome/icons/ )
Click on the icon you want (that exists within the version of Font Awesome Discourse is using). Copy its unicode value ( see http://fortawesome.github.io/Font-Awesome/icon/eye-slash/ )
Paste the unicode value in the “” next to context and put a slash in front
.badge-category i.fa.fa-lock:before { content: "\f070"; }
.category i.fa.fa-lock:before { content: '\f070'; }