docs: upgrade Keptn to Helm from a manifest installation (#2270)

Signed-off-by: odubajDT <ondrej.dubaj@dynatrace.com>
Signed-off-by: odubajDT <93584209+odubajDT@users.noreply.github.com>
Co-authored-by: Meg McRoberts <meg.mcroberts@dynatrace.com>

docs/content/en/docs/install/upgrade.md

@@ -26,6 +26,51 @@ If you installed your Keptn instance from the Manifest,
 additional steps are required to use the Helm Chart to upgrade.
 Contact us on Slack for assistance.
 
+## Upgrade to Helm from a manifest installation
+
+> **Warning**
+Upgrade to Helm from a manifest installation can cause loss
+of Keptn custom resources.
+
+Keptn v.0.7.0 and later can be installed with Helm charts;
+Keptn v.0.8.3 can only be installed with Helm.
+If you previously installed Keptn from manifests,
+you can not directly upgrade with Helm but must back up your manifests,
+then reinstall Keptn from a Helm chart and re-apply your manifests.
+
+To start the upgrade process, follow the steps below:
+
+1. To not loose all of your data, we encourage you to do a backup of the manifests,
+which you applied to the cluster (`Pods`, `Deployments`,
+`StatefulSets`, `DeamonSets`, `KeptnApps`,... ).
+After the re-installation of Keptn with Helm, you can re-apply
+these manifests and restart the Keptn deployment process.
+
+1. Completely remove your Keptn installation with the following command sequence:
+
+```shell
+your-keptn-version=<your-keptn-version>
+kubectl delete -f \
+     https://github.com/keptn/lifecycle-toolkit/releases/download/$your-keptn-version/manifest.yaml
+```
+
+1. Use Helm to install a clean version of Keptn:
+
+```shell
+helm repo add klt https://charts.lifecycle.keptn.sh
+helm repo update
+helm upgrade --install keptn klt/klt -n keptn-lifecycle-toolkit-system --create-namespace --wait
+```
+
+For information about  advanced installation options, refer to
+[Modify Helm configuration options](install.md).
+
+1. After the installation finishes, restore the manifests from you backup
+
+> **Warning** Please be aware that Keptn applications will start the deployment process from the
+beginning and the system is not guaranteed to return
+to the exact state it was in before re-installation, even if you created the backup correctly.
+
 ## Migrate from v0.6.0 to v0.7.0
 
 Keptn Version v0.7.0