Magento change dropdown attribute to multiselect

change-dropdown-attribute-to-multiselect.sql

UPDATE eav_attribute SET
entity_type_id = 4,
attribute_model = NULL,
backend_model = 'eav/entity_attribute_backend_array',
backend_type = 'varchar',
backend_table = NULL,
source_model = NULL,
frontend_model = NULL,
frontend_input = 'multiselect',
frontend_class = NULL
WHERE attribute_id = YOUR_ATTRIBUTE_ID;

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 = YOUR_ATTRIBUTE_ID;

DELETE FROM catalog_product_entity_int WHERE entity_type_id = 4 and attribute_id = YOUR_ATTRIBUTE_ID;