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.