Magento Convert Dropdown Attribute to Multiselect.sql

/*
Important! Change 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 = 165;

DELETE FROM catalog_product_entity_int WHERE attribute_id = 165;

UPDATE eav_attribute SET backend_type='varchar', frontend_input='multiselect', backend_model='eav/entity_attribute_backend_array' WHERE attribute_id = 165;