Update CoreDNS docs for beta (#7638)

* Update CoreDNS docs for beta

* Review comments

docs/tasks/administer-cluster/coredns.md

@@ -5,7 +5,7 @@ title: Using CoreDNS for Service Discovery
 min-kubernetes-server-version: v1.9
 ---
 
-{% include feature-state-alpha.md %}
+{% include feature-state-beta.md %}
 
 {% capture overview %}
 This page describes how to enable CoreDNS instead of kube-dns for service
@@ -20,15 +20,31 @@ discovery.
 
 ## Installing CoreDNS with kubeadm
 
-In Kubernetes 1.9, [CoreDNS](https://coredns.io) is available as an alpha feature and
-may be installed by setting the `CoreDNS` feature gate to `true` during `kubeadm init`:
+In Kubernetes 1.9, [CoreDNS](https://coredns.io) is available as an alpha feature, and
+in Kubernetes 1.10 it is available as a beta feature. In either case, you may install
+it during cluster creation by setting the `CoreDNS` feature gate to `true` during `kubeadm init`:
 
 ```
 kubeadm init --feature-gates=CoreDNS=true
 ```
 
 This installs CoreDNS instead of kube-dns.
 
+## Upgrading an Existing Cluster with kubeadm
+
+In Kubernetes 1.10, you can also move to CoreDNS when you use `kubeadm` to upgrade
+a cluster that is using `kube-dns`. In this case, `kubeadm` will generate the CoreDNS configuration
+("Corefile") based upon the `kube-dns` ConfigMap, preserving configurations for federation,
+stub domains, and upstream name server.
+
+Note that if you are running CoreDNS in your cluster already, prior to upgrade, your existing Corefile will be
+**overwritten** by the one created during upgrade. **You should save your existing ConfigMap
+if you have customized it.** You may re-apply your customizations after the new ConfigMap is
+up and running.
+
+This process will be modified for the GA release of this feature, such that an existing
+Corefile will not be overwritten.
+
 {% endcapture %}
 
 {% capture whatsnext %}