From a security perspective, using either of these, particularly Admin weakens the security of your blog as it is one less piece of information for a hacker to crack in order to gain access. I have security software running on all my websites and it shows me just how often hackers try to gain access using ‘admin’ or ‘administrator’ as the username. Check out my article ‘WordPress Usernames – What To Avoid’.
2) You may have entered an email address which you don’t want to use to login with, or as I have done in the past, made a spelling error!
3) You are passing your website or blog over to someone else.
Couple of important notes before you start:
An email address can only be associated with ONE user, so you will need a different temporary email address from the one you currently use. Once you have changed the username, you can easily go in and update the email address associated with it.
If you operate a FORUM on your site, check & see if it is linked to the ‘Admin’ user’ in any way. If so, you will need to reassign everything to the new user logon BEFORE you carry out the ‘delete the original admin’ step in the process.