I've had Kubuntu installed for several years, updating from one major release to another, and now I think I'd like to try to switch to Lubuntu. But since my distribution is already tuned and shaped to satisfy my needs, I'd like not to get a fresh installation, but just purge kubuntu-desktop and install lubuntu-desktop. But when I execute sudo apt-get install lubuntu-desktop, it gives me an error of the following essence:

The following packages have unmet dependencies:
  lubuntu-desktop : Depends: usb-modeswitch but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

So, I investigate the modeswitch problem and get

The following packages have unmet dependencies:
  usb-modeswitch : Depends: usb-modeswitch-data (>= 20110227-1~) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

But:

Reading state information... Done
usb-modeswitch-data is already the newest version.

So, my question is: what actions should I take in order to obtain a clean and fresh installation of Lubuntu? And also, how do I safely remove all the kubuntu-desktop-related stuff?