A lot of paranoid people have cookies disabled on their systems because they fear all sorts of terrible things will happen, which is completely false. A cookie is nothing more than a ticket or armband that is given to your browser. Each time your browser pulls up a new page, it shows us the ticket and we use that to reestablish your identity. This enables us to tell users apart and gives us the ability to customize things for you, and let you make updates without asking for passwords again. We have a more technical article about cookies that has references and stuff.
Every user account has two parts, a callsign and an email address.
Both parts must be unique. Each username and each email address can only appear once in the system. It suggests that someone else is already using that email address. Be sure and mention your old username and your requested new username - we're not mind readers. We cannot create two accounts that use the same email address.