I'm not sure exactly why the field is grayed out for you, are you certain the customization was completed through the schema designer, and not some other means?
You should look through the site\mods\majic directory and see if any file contains a code like:
MODIFY cr affected_resource
{
REQUIRED;
};
If you see this, you can remove it to stop the field from being required (if schema designer WAS used, you'll find this in wsp.mods).. If the customization was completed in schema designer, you'll also need to look in the wspcol table, and you may have to manually remove the change there. Maybe use a query like:
select * from wspcol where column_name = 'affected_resource' and table_name = 'cr'