add warning note for downgrade error

Signed-off-by: Ian Cardoso <[email protected]>

Author: osodracnai

README.md

@@ -61,6 +61,37 @@ spec:
 ``` 
 
 The upgrade controller should watch for this plan and execute the upgrade on the labeled nodes. For more information about system-upgrade-controller and plan options please visit [system-upgrade-controller](https://github.com/rancher/system-upgrade-controller) official repo.
+## Warning
+
+Kubernetes does not support downgrades of control-plane components. Starting with the 2023-07 releases ([v1.27.4+k3s1](https://github.com/k3s-io/k3s-upgrade/releases/tag/v1.27.4%2Bk3s1), [v1.26.7+k3s1](https://github.com/k3s-io/k3s-upgrade/releases/tag/v1.26.7%2Bk3s1), [v1.25.12+k3s1](https://github.com/k3s-io/k3s-upgrade/releases/tag/v1.25.12%2Bk3s1), [v1.24.16+k3s1](https://github.com/k3s-io/k3s-upgrade/releases/tag/v1.24.16%2Bk3s1)) the k3s-upgrade image used by upgrade plans will refuse to downgrade K3s, failing the plan and leaving your nodes cordoned.
+
+If you attempted it your pods in cluster should look something like this:
+```
+ubuntu@user:~$ kubectl get pods -A
+NAMESPACE        NAME                                                              READY   STATUS    RESTARTS   AGE
+kube-system      coredns-77ccd57875-9ng74                                          1/1     Running   0          19h
+kube-system      local-path-provisioner-957fdf8bc-9vwzn                            1/1     Running   0          19h
+kube-system      metrics-server-648b5df564-wzbnh                                   1/1     Running   0          19h
+kube-system      svclb-traefik-0bda8e84-hbjq8                                      2/2     Running   0          19h
+kube-system      svclb-traefik-0bda8e84-jg94l                                      2/2     Running   0          19h
+kube-system      svclb-traefik-0bda8e84-qkcs7                                      2/2     Running   0          19h
+kube-system      svclb-traefik-0bda8e84-tfhjq                                      2/2     Running   0          19h
+kube-system      traefik-64f55bb67d-4mm6s                                          1/1     Running   0          19h
+system-upgrade   apply-k3s-server-on-ip-172-31-0-16-with-7af95590a5af8e8c3-2cdc6   0/1     Error     0          9m25s
+system-upgrade   apply-k3s-server-on-ip-172-31-10-23-with-7af95590a5af8e8c-9xvwg   0/1     Error     0          14m
+system-upgrade   apply-k3s-server-on-ip-172-31-13-213-with-7af95590a5af8e8-8j72v   0/1     Error     0          18m
+system-upgrade   system-upgrade-controller-7c4b84d5d9-kkzr6                        1/1     Running   0          20m
+```
+and your nodes something like this:
+```
+NAME               STATUS                     ROLES                       AGE   VERSION
+ip-172-31-0-16     Ready,SchedulingDisabled   control-plane,etcd,master   19h   v1.27.4+k3s1
+ip-172-31-10-23    Ready,SchedulingDisabled   control-plane,etcd,master   19h   v1.27.4+k3s1
+ip-172-31-13-213   Ready,SchedulingDisabled   control-plane,etcd,master   19h   v1.27.4+k3s1
+ip-172-31-2-13     Ready                      <none>                      19h   v1.27.4+k3s1
+```
+
+You can return your node to service by uncordoning it with the command `kubectl uncordon NODE_NAME`(only lasts until system-upgrade-controller next try), or by changing the version or channel on your plan to target a release that is the same or newer than what is currently running on the cluster.
 
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