Moving site from doman to subdomain without breaking the links already shared

I recently need to move my wordpress lifestyle site from main domain (http://secretdiarybd.com) to a subdomain (http://bn.secretdiarybd.com) . Currently i have about 8000 blog posts and most of them have been shared in social network sites (facebook fan page and twitter). If i move the site to subdomain then existing links shared in the fan page will be broken. Is there any way to redirect all those to subdomain by post id or something? Such that if post id is less than 8000 then it will redirect to subdomain otherwise go to main domain. Any help will be appreciated.