The way I or Buffi described, you wipe out any previous OS. So yes you can install OS2006 again as you did OS2007, but you'll have to also install again the apps you want.

More complex setups involve dual- or multiple-booting to have two or more OSs on the tablet simultaneously, you select the one you want at boot time. Or you can save an image of your current OS to a PC or an MMC card, then install a new OS and if you want to go back you can reinstall the image of the old OS. Search in these forums for clues.