Open app/code/core/Mage/Adminhtml/Model/Sales/Order/Create.php
Check following method:
public function createOrder()
{
Check following code:
if ($oldOrder->getId()) {
$originalId = $oldOrder->getOriginalIncrementId();
if (!$originalId) {
$originalId = $oldOrder->getIncrementId();
}
$orderData = array(
'original_increment_id' => $originalId,
'relation_parent_id' => $oldOrder->getId(),
'relation_parent_real_id' => $oldOrder->getIncrementId(),
'edit_increment' => $oldOrder->getEditIncrement()+1,
'increment_id' => $originalId.'-'.($oldOrder->getEditIncrement()+1)
);
$quote->setReservedOrderId($orderData['increment_id']);
$service->setOrderData($orderData);
$oldOrder->cancel();
}
Comment out following two line:
//$quote->setReservedOrderId($orderData['increment_id']);
//$service->setOrderData($orderData);
NB: Don't modify core code. Use overwrite for modification.