Most bloggers have published content under their .wordpress.com URLs prior to purchasing a domain name via a domain mapping upgrade. The reason they do purchase domain mapping is so that all the URLs from the .wordpress.com root blog when clicked seamlessly redirect to the same content in the same posts and pages under the new domain URLs.
The page rank and authority the blog earned was earned by the .wordpress.com URLs and belongs to them. It’s not transferable and that means is the domain starts from zero. In about 4 – 6 months time the same content originally indexed under the.wordpress.com URLs, will all be re-indexed by search engines under the domain URLs. All things being equal, the blog will probably have the same page rank and authority it had prior to purchasing domain mapping.
If you use the JetPack plugin on your new WordPress.org install you may be able to get Staff to move your subscribers.