Can we switch desktop ( GNOME, KDE, etc ) within linux?
Yes you can, for example you can run following command for installing KDE desktop:
sudo apt-get install kde-standard
Or for full set of package/applications (it may take a lot of tim) you can run:
sudo apt-get install kde-full
after next login select KDE from "session" on the Login Prompt to start enjoying the Kool Desktop Environment (KDE).
I recommend if you want to try new desktop, install a fresh one of an specific derivative (e.g. Kubuntu or Mint KDE).
Yes.
If you install the new desktop environment (KDE, GNOME, LXDE, XFCE, etc.) you should see a option appear on the login splash-screen. On mint you select session, and then select your preferred environment.
This will now be your default environment.
I'm not exactly sure if this is how it normally works, but when I ran
sudo apt-get install kde-full
on Linux Mint 14 MATE, it asked whether I wanted MATE or KDE as my default.