actually it would be
Perhaps easier would be to just use the Synaptic Gui.
sudo apt-get install xubuntu-desktop
When xubuntu desktop is installed, it won't delete any of the regular Ubuntu gnome apps, and you are likely to find duplicates - such as the regular Ubuntu nautilus file manager, and also Thunar, Xubuntu's file manager for instance. So you can optionally keep some of those heavyweight apps like nautilus around, or delete them - you choice.
To get back to a "pure" Ubuntu/Kubuntu/Xfce4 environment when another one has already been installed, I find the following from psychocat very handy when you would like to totally remove one or multiple desktop environments:
Once loaded, in the lower left of the Ubuntu screen is "Options" and from there you can choose a specific desktop session (and more) to run.