Board Thread:Help Desk/@comment-6527064-20130718105554/@comment-4139585-20130718124732

You cannot rename categories because it would overload the server: a bot would have to automatically change the category in every page that has that category, and remember how big some categories can be. You can create a new category, replace the category on every page with a bot, and then delete the original category, but if a single bot was assigned to do the middle step across all of Wikia, the server would be overloaded.

I can help you do the middle step if you would like. What category do you want renamed and what to?