In addition to Jochen's words I want to add a couple of mine (don't you mind, Jochen? )

To make the downgrade use v17/

Before any actions with navi unit reset your navi - swith off the ignition, wait till navi goes to sleep (LED goes off), swith off two connectors from the back side of navi unit, wait 15-20 min, connect cables again. Of course, this is not an obligatory for the reanimation of your navi and may be you'll have success without this actions but I always do this before reanimation.

And one more small addition or confirmation of Jochen's words - after you'll insert SWdisk update process will really starts not immideatly, be patient and wait some time

Good luck