Modify Database in Multi-Site wp_usermeta table

This Should Work?

ALTER TABLE wp_usermeta
ADD COLUMN old_user_id bigint(20) unsigned;

INSERT INTO wp_usermeta (old_user_id)
SELECT u.id
FROM wp_users AS u;

INSERT INTO wp_usermeta (user_id, meta_key, meta_value)
SELECT u.id, 'wp_2_capabilities', meta_value
JOIN wp_users AS u ON u.user_id = u.old_user_id;

ALTER TABLE wp_users
DROP COLUMN old_user_id;