How to convert varchar type attribute to decimal/price type in Magento? - https://blog.magepsycho.com/how-to-convert-attribute-type-of-varchar-to-price-in-magento/

varcharToPriceConverter.php

<?php
/**
 * @author 	Raj KB <magepsycho@gmail.com>
 * @website     https://www.magepsycho.com
 *
 */
$mageFilename = 'app/Mage.php';
require_once $mageFilename;
Mage::setIsDeveloperMode(true);
ini_set('display_errors', 1);
umask(0);
Mage::app('admin');
Mage::register('isSecureArea', 1);

# EDIT HERE...Note the single quotes inside the double quotes. This is necessary unless you modify the function yourself
# Note that these attribute codes are those attributes whose type is to be changed.
$_attributeCodes = array("'shipping_cost_with_tax'", "'shipping_cost_without_tax'");

function copyVarcharToDecimal(){
	$connection			= _getConnection('core_write');
	$attributeIds		= (string) implode(',', _getAttributeIds());
	$entityTypeId		= (int) _getEntityTypeId();
	$sql				= 'SELECT * FROM ' . _getTableName('catalog_product_entity_varchar') . ' WHERE attribute_id IN ('.$attributeIds.') AND entity_type_id = '.$entityTypeId;
	$rows				= $connection->fetchAll($sql);
	$insertCount		= 1;
	$deleteCount		= 1;
	$insertOutput		= '';
	$deleteOutput		= '';
	foreach($rows as $row){
		$checkIfDecimalValueExists = _checkIfDecimalValueExists($row);
		if(!$checkIfDecimalValueExists){
			$sql = 'INSERT INTO ' . _getTableName('catalog_product_entity_decimal') . ' (`entity_type_id`,`attribute_id`,`store_id`,`entity_id`,`value`)
					VALUES (?,?,?,?,?)';
			$price = $row['value'];
			$price = trim(str_replace(',', '.', $price));
			$connection->query($sql, array($row['entity_type_id'], $row['attribute_id'], $row['store_id'], $row['entity_id'], $price));
			$insertOutput .= $insertCount . '> INSERTED::' . $connection->lastInsertId() . ' :: ' .$row['value'] . ' => ' . $price . '';
			$insertCount++;
		}
		$sql = 'DELETE FROM ' . _getTableName('catalog_product_entity_varchar') . ' WHERE value_id = ?';
		$connection->query($sql, $row['value_id']);
		$deleteOutput .= $deleteCount . '> DELETED::'.$row['value_id'].'';
		$deleteCount++;
	}
	echo '=================================================';
	echo 'INSERTED';
	echo $insertOutput;
	echo '=================================================';
	echo '=================================================';
	echo 'DELETED';
	echo $deleteOutput;
	echo '=================================================';
}

function _getTableName($tableName){
	return Mage::getSingleton('core/resource')->getTableName($tableName);
}

function _getConnection($type = 'core_read'){
	return Mage::getSingleton('core/resource')->getConnection($type);
}

function _getAttributeIds(){
	global $_attributeCodes;
	$attributeCodes = (string) implode(',', $_attributeCodes);
	$connection = _getConnection('core_read');
	$sql = "SELECT attribute_id
			FROM " . _getTableName('eav_attribute') . "
			WHERE attribute_code
			IN (
				" . $attributeCodes . "
			)";
	return $connection->fetchCol($sql);
}

function _getEntityTypeId(){
	$connection = _getConnection('core_read');
	$sql		= "SELECT entity_type_id FROM " . _getTableName('eav_entity_type') . " WHERE entity_type_code = 'catalog_product'";
	return $connection->fetchOne($sql);
}

function _checkIfDecimalValueExists($row){
	$connection = _getConnection('core_write');
	$sql		= 'SELECT COUNT(*) FROM ' . _getTableName('catalog_product_entity_decimal') . ' WHERE attribute_id = ? AND entity_type_id = ? AND store_id = ? AND entity_id = ?';
	$result		= $connection->fetchOne($sql, array($row['attribute_id'], $row['entity_type_id'], $row['store_id'], $row['entity_id']));
	return $result > 0 ? true : false;
}

#simply call as:
copyVarcharToDecimal();