I just changed my blog name but then realised that this was leading to broken links, so want to change it back. I didn’t choose to permanently disable the name as I knew if I had any problems like this I would be able to change it back. However now it is saying I can’t change it back as the blog already exists.
That 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, and retain and page rank the blog has or authority your have gained in the niche then you can keep the address and purchase the Site Redirect upgrade. http://en.support.wordpress.com/site-redirect/