Currency display in the invoice

Hi. I would need that all my invoice in $ (AUD) to be displayed in the invoice in AUD instead of just $ sign.
I have found in a previous of your post that snippet (https://gist.github.com/webtoffee-git/bf6d5ee02d1625298004bea18492f321)
that I have adapted for my case:

add_filter('wt_pklist_alter_currency_symbol','wt_change_currency_symbol',10,5);
function wt_change_currency_symbol($wc_currency_symbol,$symbols,$user_currency,$order,$price)
{
	if($user_currency='AUD')
	{
		$wc_currency_symbol=' AUD ';
	}
	return $wc_currency_symbol;
}

But it doesn’t work. Is it only working for new invoices? Or will it work for the older ones? Thanks!