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.