Magento – How to change a Simple Product to a Bundle Product

This is built-in with bundles. When setting up a new product you can set custom pricing for simple products in a bundle option / selection - to do so you have to set pricing to be 'fixed' instead of 'dynamic'.

Ok, seems like my solution haven't caused much issues on our system so far.

So, this MySQL code does actual trick:

/*
 * preparing list of simple products,
 * which are not associated with any other
 * complex products (group, bundle, configurable and so on)
 */
SET @simpleSkuList = (
    SELECT
        GROUP_CONCAT(sku)
    FROM
        catalog_product_entity
    WHERE
        type_id = 'simple'
        AND LENGTH(sku) = 8 /* our criteria to identify products, which need update */
        AND(
            entity_id NOT IN(
                SELECT DISTINCT
                    (product_id)
                FROM
                    catalog_product_super_link /* associated product ids of configurable */
                UNION
                    SELECT DISTINCT
                        (linked_product_id)
                    FROM
                        catalog_product_link /* associated product ids of grouped */
                UNION
                    SELECT DISTINCT
                        (child_id)
                    FROM
                        catalog_product_relation /* associated product ids of bundled and grouped */
            )
        )
    GROUP BY type_id
);

/* changing simple products type to be 'bundle' products */
UPDATE
    catalog_product_entity
SET
    type_id = 'bundle'
WHERE find_in_set(sku, @simpleSkuList);

We also doing the same for group products, but at the same time removing all custom options from group product itself. As well as removing all child-to-parent relations. We don't care about these relations - they will be re-created on import. But we have to keep products, since they have media files attached, which is not part of the import.