Problem Mapping a subdomain to an GAE app

I have a GAE app (Aggregate web site) at the url projectname.appspot.com Now, I bought a domain mydomain.com from LWS and I want to map the subdomain projectname.mydomain.com to projectname.appspot.com After setting all the required steps suggested by GAE, I'm able to reach my GAE app via projectname.mydomain.com; but, once the page finishes loading, instead of the new url (my hope), the old one (projectname.appspot.com) still displaying in the address bar of my browser. in advance, thanks for your help. Regards,