I'm not sure about changing the default document root automatically. You can set the default home directory location in "Basic cPanel Setup". For the document root, you need to manually change in apache config file, distill apache so the cPanel update wont overwrite it. It would be better not to touch the default cPanel settings as it can issues during update of control panel. Why are you trying to change the document root for all accounts? Are you migrating accounts from some other controlpanel to cPanel ?
Cheers!!!