Allows administrators or capable users to change the users profile url.
You probably don't have the edit_users capability (for user slug) and manage_options capability (for author base).
Eventually, check out my blog for more infos or tips (sorry guys, it's in french).
Not exactly. The author base won't be customized anymore, but each user will keep his customized author slug. To retrieve the initial author slugs, empty the field on each user edit page (only the customized ones of course).