As I've already seen several people meeting this problem, here's a quick article with the solution. When you change the DSCC registry hostname after installation, you can't access the DSCC GU anymore. Actually, the DSCC webapp needs to know where to find the DSCC directory, since it serves as the DSCC repository. So you need to change the parameter below in the DSCC web application configuration, which is located in the DSCC web.xml file, and then restart the application.

<param-name>sun.directory.dcc7.registry.url</param-name>
<param-value>ldap://new_fqdn:dscc_registry_ldap_listen_port</param-value>