function claue_change_portfolio_slug() {
    // get the arguments of the already-registered taxonomy
    $portfolio_cat_args = get_taxonomy( 'portfolio_cat' ); // returns an object

    // make changes to the args
    // in this example there are three changes
    // again, note that it's an object
    $portfolio_cat_args->show_admin_column = true;
    $portfolio_cat_args->rewrite['slug'] = 'your_new_portfolio_slug_here';
    $portfolio_cat_args->rewrite['with_front'] = false;

    // re-register the taxonomy
    register_taxonomy( 'portfolio_cat', 'your_new_portfolio_slug_here', (array) $portfolio_cat_args );
}
// hook it up to 11 so that it overrides the original register_taxonomy function
add_action( 'init', 'claue_change_portfolio_slug', 11 );