Semantically, cloud-config is really not something you can add after. cloud-config comes from Ubuntu, but the semantics are a virtual clone of the ones of AWS's cfn-init; the point of both is to inject your config during initial instance bring-up, when system config files are first being generated by the instance provisioner. You can't really run them again once you've already brought up the instance, since they'd just messily trample over their previously spewed configs without removing the previous ones first.

With cloud-config, you're basically expected to be using ephemeral (or nearly-so) instances, particularly within the context of an autoscaling group or equivalent. The lifecycle is supposed to go "[scale up], provision, configure, start services; [crash or scale down], terminate, repeat." CoreOS adds to this soft reboots for upgrades, but definitely still assumes cattle, not snowflake, instances.