2.6 is the first version supports caas model upgrade.
So 2.5 k8s controller can be upgraded to 2.6,
but 2.5 caas model will not be able to be upgraded to 2.6 correctly(because 2.5 operators do not have the knowledge of how to do upgrade).
What we can do is, kubectl delete all operator pods of that model after run `juju upgrade-model`.
2.6 is the first version supports caas model upgrade.
So 2.5 k8s controller can be upgraded to 2.6,
but 2.5 caas model will not be able to be upgraded to 2.6 correctly(because 2.5 operators do not have the knowledge of how to do upgrade).
What we can do is, kubectl delete all operator pods of that model after run `juju upgrade-model`.