When I changed a "guest" to be assigned a member from Orders page (particular), it seems to work but I didn't see downloadable link in the My Account Page. Saw that the wp_woocommerce_downloadable_product_permissions table doesn't update the user_id to reflect the above change. Its set to 0 all the time. I manually changed the user_id and I saw the downloadable link.
Perhaps this can be fixed ?