There is a fee of $5 USD to change your InsaneJournal username.
You may only rename your personal account, not an asylum you maintain.
Renaming the account only changes the username. Everything else - Self-committed [paid] or Permanently Insane account status, user privileges, personal info, friends, and other account-related data - remains the same, and will be associated with the new username. All existing comments in other journals and entries in asylums will display the new username.
You have the option to either redirect the old username to the new username or mark it as deleted. You may also choose to either have people who list you as a friend continue to list your new username as a friend, or be removed from the friends lists of all other users.
Any username that is not currently registered, any username that you own, or any username that has been purged is available for rename. If you would like to see a list of expunged usernames click here.
You first buy a rename token, which is e-mailed to you, then use that token to do the actual rename when you receive it. Rename tokens are non-refundable, but are transferrable. That is, you can buy a rename token for another user.
You do not have to create an account with your new username in order to be renamed to it. However, if you are worried that someone might take the name before you have a chance to buy and use your token, you should create an account with the new username. Be sure to use the same e-mail address and password that you use for your current account for the new one.