Alterando a Slug de um Post Type via functions.php

add_filter( 'register_post_type_args', 'alterar_slug_register_post_type_args', 10, 2 );
function alterar_slug_register_post_type_args( $args, $post_type ) {
    if ( 'slug-atual' === $post_type ) {
        $args['rewrite']['slug'] = 'nova-slug';
    }
    return $args;
}