Magento change dropdown attribute to multiselect

change-dropdown-attribute-to-multiselect.sql

-- ONLY VALID SINCE MAGENTO 1.9.3.x

-- 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 = 'text',
backend_table = NULL,
frontend_model = NULL,
frontend_input = 'multiselect',
frontend_class = NULL,
source_model = NULL
WHERE attribute_id = YOUR_ATTRIBUTE_ID;

-- Next, copy the attribute values from the old table to the new one
INSERT INTO catalog_product_entity_text ( 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;

-- 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;

/*
The value for "entity_type_id" that you’ll want to use may not always be "4" as referenced above.
You'll want to check the column "eav_entity_type" in table "eav_attribute" 
and find the entry that has the "entity_type_code" set to "catalog_product".
The "entity_type_id" for that record is the value you need to use. In most cases this value is either "4" or "10", 
but this may vary in future versions of Magento.
*/