What exactly is the upgrade option in the Tumbleweed installer?

As far as I remember, this feature can be used, among other things, to install other desktop environments in addition to the default desktop environment, for example. Is this really true?

From the installer, you can use the upgrade option to go from Leap to Tumbleweed.

You can also use it to update an older Tumbleweed. For example, I have an old Tumbleweed that I haven’t even booted for a few years. I would expect problems with a normal update, since it is so far out of date. But using the installer to update it will probably work fine.

As for installing additional desktops – yes, you could do that. But for most people is it easier to just use Yast Software Management for that. In Yast software, just use the “Patterns” view and select the desktop(s) that you wish to add.

Thank you for your reply. It is a great relief that Yast can be used to install other desktop environments without the need to use a command line. Honestly, I prefer to get used to the latter. since I use Arch or Debian Sid.

YaST will be fine for most day to day system mangement tasks. But I see you are running Tumbleweed. For keeping that up-to -date you can not use YaST. It can only be done from the CLI. As root:

zypper dup

Thanks for your reply, but I will also use this for updates as described here. SDB:Upgrade Tumbleweed - openSUSE Wiki

Absolutely, yes.
If you are upgrading though, I’d recommend that you add your additional Desktop Environments in a separate step than upgrading. Upgrading is complex enough, keep that simple so that if anything goes wrong, it’s less difficult to troubleshoot.

For new installs,
The screenshots in my openSUSE presentation slide deck describe how to install an additional (or alternative) Desktop. If you decide to add a Desktop to your system after install, you can launch YaST Software Manager and you will see something very similar to the screenshots for a new install