Different desktops can be installed together, but that can come with some issues, in particular where your application menu will be overpopulated with applications from the different desktop environments with duplicate functionality.
Can I switch to a different desktop environment seamlessly without losing data?
As indicated, one can install multiple desktop environments. "Seamless" is a subjective term. If you mean by that "without effort", then yes: it is a matter of a single apt command and then logging in to the new desktop. If, by "seamless", you mean ending up on a system as if only that other desktop was installed, then the answer is: that is more difficult, and will require some manual reconfiguration and removal of packages.
To quantitatively remove the desktop of the Ubuntu version you installed, is difficult. However, if you take note of all of the packages that are installed when you install another desktop, you will later on be able to quantitatively remove them again.
Can I swap kUbuntu with Ubuntu (losing data is fine in this case)?
Not sure why you doubt about this. It is a matter of grabbing the Ubunt installation medium, doing an install overwriting your current linux install, and then putting your data back. Probably the easiest way for a "seamless" transition in the second meaning of the word.
If swapping desktop environments is possible seamlessly, does it apply to Xubuntu, Lubuntu, MATE and other Ubuntu flavours?
Again, in the second sense of the word, it is a matter of reinstalling with the corresponding installation medium. In the first sense of the word, you can install multiple desktop environments, have them all installed simultaneously, and log in to any of them any time. The only caveat here is that you need to take note of the installed packages if you later want to remove a desktop environment.
