Personally, I think it is simply easier to simply install Kubuntu in the first place. Still, this is a good how-to for converting an existing Ubuntu install should the need arise.
In the Introduction to this book, I mentioned that there are several distributed versions of Ubuntu. These include Edubuntu and Kubuntu, both of which are Ubuntu but with a different default environment. I also introduced you to the concept of desktop environments and told you that GNOME was the default environment for Ubuntu. There is, however, another very popular desktop environment for Linux called KDE. I highly recommend that you introduce yourself to KDE and that you work with it as well. I make that same recommendation for people already working with KDE, by the way. Try out the GNOME desktop as well. You have a choice with Linux. Why not see what works best for you?
Showing you how to work with KDE is beyond the scope of this book, so I won’t spend a great deal of time on it, but I will show you how to convert your Ubuntu system to a Kubuntu system. If you want to learn more about KDE, may I suggest that you look at my book, Moving to Linux: Kiss the Blue Screen of Death Goodbye! I cover KDE in detail and the concepts you learn there will serve you well on your Kubuntu system. … Source: Linux Journel