Please see
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 );
