I copied the last firmware (still on Asus site) on a FAT32 sdcard, after I unziped it one level. When I plugged in the sdcard, instantly the message for downgrading appeared. It finished in some minutes, and then I restored root with motochopper.
Next step, I made a backup of root with Voodoo, and now I am updating again to the latest firmware to see if voodoo will bring the root access back.
I will keep you in touch.
[EDIT]
So, after updating to the latest firmware, I restored root access with voodoo.
This means, it is possible to keep root access, but please use voodoo before updating.