create config backup when upgrading to OpenDTU

Author: schlimmchen

docs/firmware/howto/upgrade_upstream.md

@@ -1,21 +1,35 @@
 # Upgrade from OpenDTU
 
-In general, one can switch seamlessly from OpenDTU to OpenDTU-OnBattery, as the
-parts and features inherited from the upstream project are kept compatible.
-This is particularly true for the settings.
+!!!note "Configuraton Backup"
+    Download a backup of your `config.json` using the Web UI before switching
+    between OpenDTU and OpenDTU-OnBattery.
+
+## Partition Layout
 
-However, when switching from OpenDTU to OpenDTU-OnBattery, performing an OTA
+When switching from OpenDTU to OpenDTU-OnBattery, performing an OTA
 update will fail, as the OpenDTU-OnBattery firmware images are too large to fit
 the upstream's partition layout. This project's partition layout for the ESP32
-flash memory is therefore different from the upstream project as of version
-2024.08.18. Follow the respective [upgrade guide](upgrade_8mb.md), which is
-also applicable when upgrading from OpenDTU to OpenDTU-OnBattery.
+flash memory is different from the upstream project as of version 2024.08.18.
+Follow the respective [upgrade guide](upgrade_8mb.md), which is also applicable
+when upgrading from OpenDTU to OpenDTU-OnBattery.
 
-In case you want to switch back to OpenDTU (without losing settings), you may
-do so by
+Note that settings are preserved when following the upgrade guide.
+
+In case you want to switch back to OpenDTU, you may do so by
 
 * performing an OTA update using the respective OpenDTU (non-factory) firmware
-  binary (unless you are using an OpenDTU-OnBattery variant without OTA
-  support).
+  binary, unless you are using an OpenDTU-OnBattery variant without OTA
+  support. This works as OpenDTU happily accepts to use the larger partitions.
 * writing the OpenDTU factory image suitable for your board using a USB
-  connection.
+  connection, which will reset the partition layout to the upstream version.
+
+## Settings
+
+In general, one can switch seamlessly from OpenDTU to OpenDTU-OnBattery, as the
+parts and features inherited from the upstream project are kept compatible.
+This is particularly true for the settings. Some settings are converted when
+upgrading from OpenDTU.
+
+Moving back to OpenDTU from OpenDTU-OnBattery is **not** seamless, so you
+should download a backup of your `config.json` file from the Web UI **before**
+upgrading to OpenDTU-OnBattery, in case you wish to return to OpenDTU.