Magento Change EAV Attribute type

change_eav_attribute_type.sql

#
# Change EAV Attribute Type from dropdown to multipleselect
# - copy old attribute values to new table
#

dropdown attribute_id = 137

UPDATE eav_attribute SET
backend_model = 'eav/entity_attribute_backend_array',
frontend_input = 'multiselect',
backend_type = 'varchar',
source_model = NULL WHERE eav_attribute.attribute_id = 137;

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 = 137;

DELETE FROM catalog_product_entity_int WHERE attribute_id = 1;