Cannot yum update after reinstallation of the redhat

My redhat was re-installed several times before. I did not check the software update at all which leads to the current situation, that is, my linux box cannot update using yum command. Whenever I type yum update, I got the prompt saying "Your redhat is not registered with RHN". After calling redhat customer service this problem is marked as solved. Here is the simple solution. Just log into the rhn.redhat.com, and delete the system you have registered before, or un-entitle it first and then delete it. After that, run the rhn_register from the terminal or click the redhat software updater and register my machine again.