How to change a product dropdown attribute to a multiselect in Magento using Database Query ?

UPDATE eav_attribute SET
entity_type_id = '10', // set here entity_type_id  from eav_attribute
attribute_model = NULL,
backend_model = 'eav/entity_attribute_backend_array',
backend_type = 'varchar',
backend_table = NULL,
frontend_model = NULL,
frontend_input = 'multiselect',
frontend_class = NULL
WHERE attribute_id = '1100'; // set here attribute_id  from eav_attribute


INSERT INTO catalog_product_entity_varchar ( entity_type_id, attribute_id, store_id, entity_id, value)
SELECT entity_type_id, attribute_id, store_id, entity_id, value
FROM catalog_product_entity_int
WHERE attribute_id = 1100; // set here attribute_id  from eav_attribute


DELETE FROM catalog_product_entity_int
WHERE entity_type_id = 10 and attribute_id = 1100; // set here attribute_id  from eav_attribute