function change_existing_currency_symbol( $currency_symbol, $currency ) {
    switch( $currency ) {
        case 'AUD': $currency_symbol = 'AUD$'; break;
    }
    return $currency_symbol;
}
add_filter('woocommerce_currency_symbol', 'change_existing_currency_symbol', 10, 2);

　　代码注释：将澳元的货币符号从默认的$改为AUD$。