function forcesendmail ( $order_id, $old_status, $new_status ){
$order = new WC_Order($order_id);
if( $new_status == "processing" ) {
WC()->mailer()->emails['WC_Email_Customer_Processing_Order']->trigger($order_id);
}
}
add_action( 'woocommerce_order_status_changed', 'forcesendmail', 99, 3 );<select id="order_status" name="order_status" >
<option value="wc-pending">Pending payment</option>
<option value="wc-processing">Processing</option>
<option value="wc-on-hold" selected="selected">On hold</option>
<option value="wc-completed">Completed</option>
<option value="wc-cancelled">Cancelled</option>
<option value="wc-refunded">Refunded</option>
<option value="wc-failed">Failed</option>
</select>
$order = wc_get_order( $order_id );
if($order){
$order->update_status( 'pending', '', true );
}$order = new WC_Order($order_id);
$order->update_status('pending', 'order_note'); // order note is optional, if you want to add a note to order