First, update the attribute input type to multiselect:
UPDATE eav_attribute SET entity_type_id = ‘4‘, 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 = ‘YOUR_ATTRIBUTE_ID_HERE‘;
Next, copy the attribute values from the old table to the new:
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_HERE;
Finally, remove the old values or they will conflict with the new setup (the old values will load, but Magento will save new values to the varchar table):
DELETE FROM catalog_product_entity_int WHERE entity_type_id = 4 and attribute_id = YOUR_ATTRIBUTE_ID_HERE;
How to change a product dropdown attribute to a multiselect in Magento,布布扣,bubuko.com
How to change a product dropdown attribute to a multiselect in Magento
原文：http://www.cnblogs.com/fengliang/p/3806674.html