mbbackus This is very tricky, since between each version, several changes are made in the OS. Some of these changes include subtle operations, such as installing packages in a particular order (this was the case e.g. for updating to PHP7.1). Moreover, if you've done some customizations, an automatic process could easily break your changes. This is why there's no automatic process to get these changes into an existing image.
What you can try (if you know what you do!):
sudo apt-get update && sudo apt-get upgrade -y to update Raspbian to the last version,
- update Moodle, as indicated here,
- update the MoodleBox plugin, just like any other Moodle plugin,
- look in the MoodleBox release notes for any changes between your current version and the latest, and
- apply manually these changes (again: tricky!).
Myself, when I want to upgrade to a newer MoodleBox, I simply backup the Moodle courses and restore them in the new image. If your setup is more complex, you could try to export the Moodle database and
moodledata folder, then restore them on the new image.