One of the great things about Twitter is the ability to change your username without losing your friends and mentions. The downside of this is that it frees up your old name. I recently saw someone change their name, only to have someone else take the old name and impersonate them.
To keep this from happening follow this very simple advice. Change your twitter username, and then immediately register a new account with the old name using a separate (most likely defunct) email address. Then post a message with the old name directing users to the new account. This way you ensure protection of your identity.