You can't delete root, but you can delete any other admin user...
I'm not sure what you are trying to do, but you could leave yourself with some unix permissions issues.
To be safe you might use /Applicaitons/Utilties/NetInfo Manager to assign your new user the UID # used by the previous admin user. That way all of the previous admin's files (unix permission) will be inherited by the new Admin users.
When you delete the old user is will create a compressed archive of that users home directory... You can delete that later, but you might keep it around for a while just in case you need to retreive something.