Recently, I noticed that there was a Zorin Upgrade app present in the start menu. Taking a look at https://zorin.com, I understood that it would allow me to upgrade my 16.2 OS to 16.3 without data loss, though I should back up my drive just to be certain. That's also why I chose to wait until I would have time to make a full system backup (I'm currently quite busy).
However, when checking the system settings, I noticed that the upgrade may have already occured – although I did not press any upgrade button, neither in the Zorin Upgrader nor in the System Updater! I also never ran the dist-ugrade command in the terminal.
Did Zorin auto-upgrade to 16.3 without my interference? Or is it a display error?
You see, the Zorin Upgrader still says that the 16.3 upgrade is available (instead of: “You're already using the latest version.”)
Can I safely try to manually upgrade to 16.3 (I'm considering to buy Pro)?
The Upgrader tool is for the Zorin Direct Upgrade option. This is for either upgrading to Pro or upgrading the Release.
A release would be Zorin 12, Zorin 15, Zorin 16.
A point release would be Zorin 16.1, Zorin 16.2 or Zorin 16.3
The Direct Upgrade path is not related to a Point Release. It is for Major Releases only, not point releases.
The difference between dist-upgrade and full-upgrade:
Running the dist-upgrade command will upgrade all packages that have upgrades available as long as files do not need to be removed or deleted to do so.
Running the full-upgrade command will upgrade all packages that have upgrades available even if files need to be removed or deleted to do so.
Neither of these commands perform the action that the do-release-upgrade command will do.
The do-release-upgrade command will not upgrade your installed packages. That must be done with the full-upgrade command first. The do-release-upgrade can then upgrade your Distributions release to the next major release.
Why is this all so confusing?
I think it really comes down to things being poorly named over the years.
On Windows OS, all updates and upgrades are just called "updates."
In Linux, "updates" refer to getting the latest sources and "upgrades" refers to getting the latest packages.
Because a lot of Windows users also use Linux, they carry over the habit of calling "upgrades" as "updates" making the words interchangeable when they shouldn't be.
Then we have different things being "upgraded": The system, the packages or the release.
To finally get to answering your question:
Your system is safe and in the clear. You may have run a regular upgrade without realizing that you did at the time. But you are safely on Zorin OS 16.3
You won't be able to perform a Direct Upgrade unless you choose Zorin Pro or Zorin OS 17 is released. So the Zorin Upgrader Tool can be ignored until you need it for either of those Two Options.
All other updates/upgrades can be performed in the Software Store, Software Updater or the terminal.