TL;DR: Do a fresh installation of Kubuntu OR keep both the desktop environments.
Your title includes the word "safely" which indicates you don't want to break things, so doing a fresh install is the best option.
- Backup you entire
/home directory.
- Download the Kubuntu ISO for your version of Ubuntu. Given that you have focal fossa, the Kubuntu 20.04 ISO should work for you. Download the ISO labelled Desktop Image from here.
- Download balenaEtcher. Get the x64 Linux version. Extract the zip. In a terminal, go into the extracted zip folder. Once you're there, if you do
ls, you should see a file that ends in .AppImage. Do chmod +x balenaEtcher-1.7.3-x64.AppImage.
- Run balenaEtcher with
./balenaEtcher-1.7.3-x64.AppImage.
- Find a flash drive that is bigger than 8GB in capacity, and put it on your computer. All data on this flash drive will be destroyed.
- Select the downloaded ISO, and select the flash drive. Hit Flash.
- Once it's done, reboot your computer, enter the boot menu, and select the name of the USB Drive.
- Go through the installation; configure it; install.
You want the interface of KDE plasma, right? So, why remove GNOME?
Removing a desktop environment (especially the default one) is not at all recommended. If Ubuntu used the Vanilla GNOME (gnome-session) then the situation was different. But Ubuntu uses a modified version of GNOME creating things more difficult. Your idea of switching to KDE is good, but removing GNOME is silly.
Ubuntu depends on the GNOME desktop. Uninstalling GNOME will break many things your whole distribution! Many application like Firefox, Gnome disks and so on does not depend on the ubuntu-desktop package. So, removing it is not enough.
Should I remove ubuntu-desktop?
NO! Removing Ubuntu Desktop will not remove the entire GNOME and will break many things. Ubuntu Desktop is just a meta-package that depends on GNOME Desktop. We can say that this meta-package was created to ensure that GNOME doesn't get easily removed. If you run apt remove ubuntu-desktop --simulate you'll find that only the package will be removed, its dependencies would be marked for removal, which would be removed when you'll run apt autormeove.
In short, removing this package won't remove GNOME and will do additional package breakage.
N.B: You won't be able to easily install Ubuntu desktop again after purging this package because of several dependency issues.
I still want to purge GNOME and install KDE plasma.
Warning: Using this method in a graphical environment with GNOME shell running, may caus several issues, consider doing this in a recovery shell or TTY
It is completely up to you if you want to give it a shot, then you can follow this answer given by N0rbert and purge GNOME:
sudo apt update
sudo apt purge adwaita-icon-theme gedit-common gir1.2-gdm-1.0 \
gir1.2-gnomebluetooth-1.0 gir1.2-gnomedesktop-3.0 gir1.2-goa-1.0 \
gnome-accessibility-themes gnome-bluetooth gnome-calculator gnome-calendar \
gnome-characters gnome-control-center gnome-control-center-data \
gnome-control-center-faces gnome-desktop3-data \
gnome-font-viewer \
gnome-initial-setup gnome-keyring gnome-keyring-pkcs11 gnome-logs \
gnome-mahjongg gnome-menus gnome-mines gnome-online-accounts \
gnome-power-manager gnome-screenshot gnome-session-bin gnome-session-canberra \
gnome-session-common gnome-settings-daemon gnome-settings-daemon-common \
gnome-shell gnome-shell-common gnome-shell-extension-appindicator \
gnome-shell-extension-desktop-icons gnome-shell-extension-ubuntu-dock \
gnome-startup-applications gnome-sudoku gnome-system-monitor gnome-terminal \
gnome-terminal-data gnome-themes-extra gnome-themes-extra-data gnome-todo \
gnome-todo-common gnome-user-docs gnome-user-docs-ru gnome-video-effects \
language-pack-gnome-en language-pack-gnome-en-base language-pack-gnome-ru \
language-pack-gnome-ru-base language-selector-gnome libgail18 libgail18 \
libgail-common libgail-common libgnome-autoar-0-0 libgnome-bluetooth13 \
libgnome-desktop-3-19 libgnome-games-support-1-3 libgnome-games-support-common \
libgnomekbd8 libgnomekbd-common libgnome-menu-3-0 libgnome-todo libgoa-1.0-0b \
libgoa-1.0-common libpam-gnome-keyring libsoup-gnome2.4-1 libsoup-gnome2.4-1 \
nautilus-extension-gnome-terminal pinentry-gnome3 yaru-theme-gnome-shell
sudo apt purge gnome-getting-started-docs gnome-getting-started-docs-ru*
sudo apt autopurge
And then:
sudo apt install kubuntu-desktop^
So what is the conclusion?
Stick to GNOME or do a fresh installation if Kubuntu.
