Site titles (Blog Names) and optional Taglines located here > Settings > General can easily be changed at any time.
If what you want to do is change the blog address ie. the URL this option will permanently disable the address and you will not be able to reuse it. Only select this option if you are sure you will not use the address again. If you want to redirect traffic to the new blog, you can keep the address and purchase the Site Redirect upgrade. http://en.support.wordpress.com/changing-blog-address/