diff --git a/README.md b/README.md
index 0b49087..e644336 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@ This serves to demonstrates how to:
 2. Partition and prepare UEFI/GPT disks for a minimal OSTree host system
 3. Generate OSTree repository in a empty filesystem
 4. Integrate OSTree with GRUB2 bootloader
+5. Upgrade existing installation with a rootfs image
 
 # Usage
 
@@ -44,3 +45,11 @@ This serves to demonstrates how to:
    $ chmod +x ostree.sh
    $ sudo OSTREE_DEV_SCSI=scsi-360022480c22be84f8a61b39bbaed612f ./ostree.sh install
    ```
+   
+5. **Upgrade existing installation:**
+   
+   While booted into a OSTree system, use:
+   
+   ```console
+   $ sudo ./ostree.sh upgrade
+   ```
diff --git a/ostree.sh b/ostree.sh
index 3ee3cc7..12d1de8 100644
--- a/ostree.sh
+++ b/ostree.sh
@@ -3,20 +3,32 @@ set -x
 set -u
 set -e
 
-# [ENVIRONMENT]: REQUIRED
-# OSTREE_DEV_SCSI=
+# [ENVIRONMENT]: OPTIONS
+function ENV_OPTS_CREATE {
+    # Required
+    # - OSTREE_DEV_SCSI (for install)
 
-# [ENVIRONMENT]: CONFIGURABLE
-export OSTREE_DEV_DISK=/dev/disk/by-id/${OSTREE_DEV_SCSI}
-export OSTREE_DEV_BOOT=${OSTREE_DEV_DISK}-part1
-export OSTREE_DEV_HOME=${OSTREE_DEV_DISK}-part2
-export OSTREE_DEV_ROOT=${OSTREE_DEV_DISK}-part3
-export OSTREE_SYS_ROOT=/mnt
-export OSTREE_SYS_BUILD=/tmp/rootfs
+    # Configurable
+    export OSTREE_DEV_DISK=${OSTREE_DEV_DISK:=/dev/disk/by-id/${OSTREE_DEV_SCSI}}
+    export OSTREE_DEV_BOOT=${OSTREE_DEV_BOOT:=${OSTREE_DEV_DISK}-part1}
+    export OSTREE_DEV_HOME=${OSTREE_DEV_HOME:=${OSTREE_DEV_DISK}-part2}
+    export OSTREE_DEV_ROOT=${OSTREE_DEV_ROOT:=${OSTREE_DEV_DISK}-part3}
+    export OSTREE_SYS_ROOT=${OSTREE_SYS_ROOT:=/mnt}
+    export OSTREE_SYS_BUILD=${OSTREE_SYS_BUILD:=/tmp/rootfs}
+}
+
+# [ENVIRONMENT]: DEPENDENCIES
+# | Todo: add persistent Pacman cache
+function ENV_DEPS_CREATE {
+    # Skip in OSTree as filesystem is read-only
+    if ! grep -q ostree /proc/cmdline; then
+        pacman --noconfirm --needed -S $@
+    fi
+}
 
 # [DISK]: PARTITIONING (GPT+UEFI)
 function DISK_CREATE_LAYOUT {
-    pacman --noconfirm --needed -S parted
+    ENV_DEPS_CREATE parted
     umount --lazy --recursive ${OSTREE_SYS_ROOT} || :
     parted -a optimal -s ${OSTREE_DEV_DISK} -- \
         mklabel gpt \
@@ -28,7 +40,7 @@ function DISK_CREATE_LAYOUT {
 
 # [DISK]: FILESYSTEM (ESP+EXT4)
 function DISK_CREATE_FORMAT {
-    pacman --noconfirm --needed -S dosfstools e2fsprogs
+    ENV_DEPS_CREATE dosfstools e2fsprogs
     mkfs.vfat -n SYS_BOOT -F 32 ${OSTREE_DEV_BOOT}
     mkfs.ext4 -L SYS_ROOT -F ${OSTREE_DEV_ROOT}
     mkfs.ext4 -L SYS_HOME -F ${OSTREE_DEV_HOME}
@@ -42,7 +54,7 @@ function DISK_CREATE_MOUNTS {
 
 # [OSTREE]: INITIALIZATION
 function OSTREE_CREATE_REPO {
-    pacman --noconfirm --needed -S ostree wget which 
+    ENV_DEPS_CREATE ostree wget which 
     ostree admin init-fs --sysroot=${OSTREE_SYS_ROOT} --modern ${OSTREE_SYS_ROOT}
     ostree admin os-init --sysroot=${OSTREE_SYS_ROOT} archlinux
     ostree init --repo=${OSTREE_SYS_ROOT}/ostree/repo --mode=bare
@@ -50,13 +62,12 @@ function OSTREE_CREATE_REPO {
 }
 
 # [OSTREE]: CONTAINER
-# | Todo: add persistent Pacman cache
 # | Todo: delete /etc in Contailerfile (https://github.com/containers/podman/issues/20001)
 # | Todo: use tar format (`podman build -f Containerfile -o dest=${OSTREE_SYS_BUILD}.tar,type=tar`)
 function OSTREE_CREATE_IMAGE {
     # Add support for overlay storage driver in LiveCD
     if [[ $(df --output=fstype / | tail -n 1) = "overlay" ]]; then
-        pacman --noconfirm --needed -S "fuse-overlayfs"
+        ENV_DEPS_CREATE fuse-overlayfs
         export TMPDIR="/tmp/podman"
         PODMAN_ARGS=(
             --root ${TMPDIR}/storage
@@ -65,7 +76,7 @@ function OSTREE_CREATE_IMAGE {
     fi
 
     # Create rootfs directory (workaround: `podman build --output local` doesn't preserve ownership)
-    pacman --noconfirm --needed -S podman
+    ENV_DEPS_CREATE podman
     podman ${PODMAN_ARGS[@]} build -f Containerfile -t rootfs
     rm -rf ${OSTREE_SYS_BUILD}
     mkdir ${OSTREE_SYS_BUILD}
@@ -86,7 +97,7 @@ function OSTREE_DEPLOY_IMAGE {
 function BOOTLOADER_CREATE {
     grub-install --target=x86_64-efi --efi-directory=${OSTREE_SYS_ROOT}/boot/efi --removable --boot-directory=${OSTREE_SYS_ROOT}/boot/efi/EFI --bootloader-id=archlinux ${OSTREE_DEV_BOOT}
     
-    export OSTREE_SYS_PATH=$(ls -d ${OSTREE_SYS_ROOT}/ostree/deploy/archlinux/deploy/*|head -n 1)
+    export OSTREE_SYS_PATH=$(ls -d ${OSTREE_SYS_ROOT}/ostree/deploy/archlinux/deploy/* | head -n 1)
     
     rm -rfv ${OSTREE_SYS_PATH}/boot/*
     mount --mkdir --rbind ${OSTREE_SYS_ROOT}/boot ${OSTREE_SYS_PATH}/boot
@@ -98,13 +109,36 @@ function BOOTLOADER_CREATE {
     umount -R ${OSTREE_SYS_ROOT}
 }
 
-# Tasks to perform
-DISK_CREATE_LAYOUT
-DISK_CREATE_FORMAT
-DISK_CREATE_MOUNTS
+# [CLI]: TASKS FINECONTROL
+case ${1:-} in
+    "install")
+        ENV_OPTS_CREATE
+
+        DISK_CREATE_LAYOUT
+        DISK_CREATE_FORMAT
+        DISK_CREATE_MOUNTS
+
+        OSTREE_CREATE_REPO
+        OSTREE_CREATE_IMAGE
+        OSTREE_DEPLOY_IMAGE
+
+        BOOTLOADER_CREATE
+        ;;
+
+    "upgrade")
+        if [[ ! -d "/ostree" ]]; then
+            exit 0
+        fi
+
+        export OSTREE_DEV_SCSI=
+        export OSTREE_SYS_ROOT=/
+        ENV_OPTS_CREATE
 
-OSTREE_CREATE_REPO
-OSTREE_CREATE_IMAGE
-OSTREE_DEPLOY_IMAGE
+        OSTREE_CREATE_IMAGE
+        OSTREE_DEPLOY_IMAGE
+        ;;
 
-BOOTLOADER_CREATE
+    *)
+        echo "Usage: ostree.sh [install|upgrade]"
+        ;;
+esac