Can I change my Ubuntu Desktop into a different flavour (like Kubuntu)?
The main difference between main Ubuntu, Xubuntu, Kubuntu, and Lubuntu is the Desktop Environment. You can install one of these DEs onto your system with the following commands:
Ubuntu:
sudo apt-get install ubuntu-desktop
Kubuntu:
sudo apt-get install kubuntu-desktop
Xubuntu:
sudo apt-get install xubuntu-desktop
Lubuntu:
sudo apt-get install lubuntu-desktop
Ubuntu GNOME:
sudo apt-get install ubuntu-gnome-desktop
From Ubuntu 17.10 GNOME became the default desktop environment/shell/manager.
So if you're running Ubuntu 17.10 forwards (including current 18.04), you can install the default desktop:
$ sudo apt install ubuntu-desktop
To remove any of these, run the command that you used to install but instead ofinstall say purge (i.e., sudo apt-get purge kubuntu-desktop). Then you also need to run this command to complete the remove:
sudo apt-get autoremove --purge
The autoremove command removes all the packages that were installed. The way these installs work is that there is one meta-package that just depends on every package that is required for that Desktop Environment. Thus the extra packages aren't removed when the meta package is, and you need to remove them with autoremove.
This will install the Desktop Environment and most programs for each of the flavors of Ubuntu, though sometimes there are programs that are not installed by this. An install switched by using these commands will not be the same as a clean install of the Ubuntu flavor, though it should be pretty close. There is also no limit as to having more then one of these Desktop Environments. I personally kept both ubuntu-desktop and ubuntu-gnome-desktop on my computer for a while just to compare.
I do not know about a changing to/from a Ubuntu Studio install, though I am pretty sure it isn't as easy as just changing the Desktop Environment.
You can also keep them all, and choose which one you want to log in to. That's a good way to compare, just switch every time you boot. Every file you're created will be in the same place, every application you installed for one flavor will be available in the others.
Ubuntu Desktop Flavours
https://www.ubuntu.com/download/flavours
Flavours:
Default (Gnome (17.10 - present)) ubuntu-desktop
Budgie (simple/elegant) budgie-desktop
Kubuntu (KDE Plasma) kubuntu-desktop
Kylin (elegant Chinese) ubuntukylin-desktop
Lubuntu (LXQt - light/fast) lubuntu-desktop
MATE (GNOME 2 fork) ubuntu-mate-desktop
Studio (Multimedia) ubuntustudio-desktop
Xubuntu (XFce - light/configurable) xubuntu-desktop
Install Alternative Desktop Flavour:
(you can install multiple Desktop Flavours, then switch them as required)
sudo apt install [flavour]
Example:
sudo apt install budgie-desktop
Reboot after installing new flavour, then choose that new Desktop Flavour by clicking the Ubuntu icon above the password field on the login screen.
If you want to remove old flavours, make sure you're logged into a session that's not using the flavour you want to remove, then:
sudo apt remove --purge [flavour]
Example:
sudo apt remove --purge ubuntu-desktop