The reason we 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 post under the new domain URLs.
The pagerank and authority the blog earned was earned by the .wordpress.com URLs and belongs to them. It’s not transferable. so what that means is the domain starts from zero. In about 4 – 6 months time the content will all be re-indexed by search engines under the domain URLs and all things being equal the blog will probably have the same pagerank and authority it had prior to purchasing domain mapping. http://en.support.wordpress.com/domain-mapping/register-domain/
If you do not renew your domain mapping then all those domain URL’s are broken and the blog reverts to the old .wordpress.com URLs. Then all those broken links are there for months until the search engines like Google index them again under the .wordpress.com URLs and clear their caches (3 – 6 months).
You can remove the domain mapping upgrade currently in place but if you do then all the links will revert back to the .wordpress.com URLs and all indexed links will produce 404 (page not found) broekn links errors. It will take about 3 - 6 months time for Google to clear their cache.
You can purchase another domain mapping upgrade and apply it to that same .wordpress.com sub-domain blog. However, then there will be 3 sets of links pointing to the same content and first two sets of links to indexed content will be broken 404 (page not found) links for at least 3 - 6 months.