I had previously followed these instructions to install and subsequently uninstall KDE using these commands:
sudo qubes-dom0-update @kde-desktop-qubes
&
sudo dnf remove kdelibs plasma-workspace
In which case if one tries to install KDE again, it will fail with the message that the ‘KDE’ group is already installed. Simply reinstalling kdelibs and plasma-workspace leaves KDE in a weird and incomplete state (e.g. no taskbar).
There might be an elegant way to solve this that I do not know. However, here is how I solved it in R4.0.4 with success based on this post:
sudo qubes-dom0-update apper breeze-icon-theme colord-kde dolphin kcm_systemd kde-runtime kde-settings-pulseaudio kde-style-breeze kdelibs kdeplasma-addons kdialog khelpcenter khotkeys kinfocenter kmenuedit kmix konsole5 kscreen ksysguard kwin phonon-qt5-backend-gstreamer plasma-breeze plasma-desktop plasma-desktop-doc plasma-pa plasma-user-manager plasma-workspace polkit-kde qt5-qdbusviewer sddm sddm-breeze sddm-kcm sni-qt
…followed by the exact same command using the --action=reinstall parameter. Funny enough, this did not only reinstall KDE but also made features (like HiDPI in SSDM) work that I previously failed to get working.