You can use the transfer a blog feature.
Every blog has a sole owner and it is the person with the username account who registered the blog under that username account and associated email address.
To transfer a blog from one username account A to another username account B you must first log in under the username account A that owns the blog you want to transfer to the other username account.
Then you add the other username account B as an Admin of the blog.
adding users http://en.support.wordpress.com/adding-users/
user roles http://en.support.wordpress.com/user-roles/
Then you A can use this process to transfer the blog to that username account B instead http://en.support.wordpress.com/moving-a-blog/#transferring-your-blog-to-another-user-or-account