Note for cleaning after downgrade

master/getting-started/kubernetes/upgrade/downgrade.md

@@ -9,6 +9,12 @@ Under some circumstances, you may need to perform a downgrade and return your
 cluster to the previous version of {{site.prodname}}. If you need to downgrade
 you should do so as soon as possible to avoid an outage.
 
+> **Important**: After downgrading or aborting the migration it is necessary
+> to delete the previously migrated [etcd](delete)
+> or [Kubernetes API](delete)
+> data before re-running the migration.
+{: .alert .alert-info}
+
 The downgrade procedure varies according to how you originally installed
 {{site.prodname}} and your datastore type.