docs: add redeploy instructions for CSI NFS driver in migration guide (PSCLOUD-2)

docs/user/NFSProvisionerUpdate-v9.md

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+
+#  Migration Guide: v9.0.0
+
+This guide assumes you are migrating a viya4-deployment (e.g., `v8.2.1`) to a newer version (e.g., `v9.0.0`) using the latest viya4-deployment baseline that includes the `csi-driver-nfs`.
+
+##  Prerequisites
+
+- Ensure you have **cluster admin access**.
+- The **NFS server** used in the existing setup must be retained and accessible.
+- All **PVs and PVCs** should be backed up as a precaution.
+
+##  Migration Steps
+
+###  Backup Existing Environment (Manual Execution)
+
+Trigger a manual backup of your running viya4-deployment:
+
+```bash
+kubectl create job --from=cronjob/sas-scheduled-backup-all-sources manual-backup-$(date +%s) -n <viya4-namespace>
+````
+
+Fatch the backup ID
+
+```bash
+kubectl describe job <backup-job-name> -n va-viya | grep "sas.com/sas-backup-id"
+````
+
+###  Verify the backup job has completed successfully:
+
+```bash
+kubectl get jobs \
+  -L "sas.com/sas-backup-id,sas.com/backup-job-type,sas.com/sas-backup-job-status,sas.com/backup-persistence-status" -n viya_namespace_name
+```
+###  Stop the viya4-deployment
+
+Stop the SAS viya4 environment using the cron job:
+
+```bash
+kubectl -n <viya4-namespace> create job --from=cronjob/sas-stop-all stopdep-<date +%s>
+```
+
+**Example:**
+
+```bash
+kubectl -n viya4 create job --from=cronjob/sas-stop-all stopdep-22072025
+```
+###  Delete Old NFS Provisioner Components
+
+Remove the `sas` StorageClass:
+For SAS viya4 environments deployed on Google Cloud Platform (GCP), the legacy `pg-storage` StorageClass must be deleted.
+
+```bash
+kubectl delete storageclass sas
+```
+
+Delete the namespace used by the legacy provisioner (typically `nfs-client`):
+
+```bash
+kubectl delete namespace nfs-client
+```
+
+###  Deploy New viya4 Environment with CSI Driver
+
+Update your DaC baseline to install the CSI NFS driver
+
+To install/upgrade baseline dependencies only using "Docker"
+
+  ```bash
+  docker run --rm \
+    --group-add root \
+    --user $(id -u):$(id -g) \
+    --volume $HOME/deployments:/data \
+    --volume $HOME/deployments/dev-cluster/.kube/config:/config/kubeconfig \
+    --volume $HOME/deployments/dev-cluster/dev-namespace/ansible-vars.yaml:/config/config \
+    --volume $HOME/.ssh/id_rsa:/config/jump_svr_private_key \
+    viya4-deployment --tags "baseline,install"
+  ```
+
+To install/upgrade baseline dependencies only using "ansible"
+
+  ```bash
+  ansible-playbook \
+    -e BASE_DIR=$HOME/deployments \
+    -e KUBECONFIG=$HOME/deployments/.kube/config \
+    -e CONFIG=$HOME/deployments/dev-cluster/dev-namespace/ansible-vars.yaml \
+    -e JUMP_SVR_PRIVATE_KEY=$HOME/.ssh/id_rsa \
+    playbooks/playbook.yaml --tags "baseline,install"
+  ```
+If you have redeployed **viya4-deployment** using the [9.0.0 release](https://github.com/sassoftware/viya4-deployment/releases/tag/v9.0.0), which includes CSI NFS driver support, no additional action is required.
+
+However, if you have only updated the viya4-deployment baseline without redeploying viya4, you will need to manually start the viya4 environment using the following command:
+
+```bash
+kubectl -n <viya4-namespace> create job --from=cronjob/sas-start-all startdep-<date +%s>
+```
+
+>  **Important Note:** You do **not** need to restore from backup, as the NFS server path to the PVs remains the same. The CSI driver will reuse existing PVs and directories automatically.
+
+###  Post-Migration Steps
+
+*  Confirm all PVCs are **bound and mounted correctly** in the new viya4-deployment.
+*  Validate **data availability** and application functionality.
+
+---
+
+###  Notes
+
+* The **CSI NFS driver** offers improved compatibility with newer Kubernetes versions and is the **recommended** provisioner going forward.
+* Avoid reusing the old Helm release metadata (`meta.helm.sh/*`) to prevent installation or upgrade conflicts.
+