Aha! Well, that's an excellent hint -- when I went to "unlock" widgets, that function is not on my desktop. I never noticed that I don't have it any more, after the transition. So, what package provides the locking and unlocking of the widgets?
EDIT: With a little google help, I suspected I was missing some plasma packages. I discovered what is missing this way:
don@dibl-patience:~$ sudo apt install -s plasma-widgets-addons
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
libqt5webengine5 qml-module-qtwebengine qml-module-qtwebkit
The following NEW packages will be installed:
libqt5webengine5 plasma-widgets-addons qml-module-qtwebengine qml-module-qtwebkit
0 upgraded, 4 newly installed, 0 to remove and 1 not upgraded.
Inst qml-module-qtwebkit (5.212.0~alpha4-4 Debian:unstable [amd64])
With these packages installed, and if I put the cursor over the pager on the panel, the mouse wheel will turn the cube.
But, the switching used to happen on the desktop itself, not only on the panel. And that is the way my other system still works. So there is more to learn on this topic. And I still don't have the "Lock" and "Unlock" widgets function -- so there is more fixing to do on this system.
Vielen dank, @der_bud!