how to alter args of a custom post type

Question

First of all, query_var must be true or a string if you want any sort of front end display to work, whether or not you use pretty permalinks.

The second part is rewrite has to at least be true. Note that valid values for rewrite are either boolean true/false, or an array. This:

$args['rewrite'] = 'my-cool-retailer';

… will enable rewrites, but it won’t set the slug to my-cool-retailer, because a string is not a valid value here, it just gets interpreted as true, and your slug will be wc_product_retailer.

If you want to set a different slug, you need to make rewrite an array with key slug:

$args['rewrite']['slug'] = 'my-cool-retailer';

As for has_archive, that’s true by default, so that works without changing anything but query_var. You can also pass a string if you want something other than wc_product_retailer to be the archive slug.

So, using your first code block and then setting those two values in the filter works for me when I test on a default install:

function change_post_types_slug( $args, $post_type ) {
    if ( 'wc_product_retailer' === $post_type ) {
        $args['query_var'] = true;
        $args['rewrite']['slug'] = 'my-cool-retailer';
   }
   return $args;
}
add_filter( 'register_post_type_args', 'change_post_types_slug', 10, 2 );

One other thing you may want to add is with_front, which is true by default. This will add any static prefix from your post permalink to your CPT, which is usually not desirable.

$args['rewrite']['with_front'] = false;