You can change display name freely. Nickname will be displayed if your display name (the "name" field in your profile) does not match it. Nickname will not be user changeable, once it's set it can only be changed by an admin. We're allowing people to freely edit it until the 20th since we just introduced the field, but after that you will not be able to change it.
The idea is to have some sort of indicator in which to identify you by. This used to be the username field, but now that it's obvious we're going to be attacked for the foreseeable future, I decided to implement a new profile field to serve the purpose that wasn't involved in the login process at all.