I can confirm that you can always go back to the state it was when you took the backup, even if it is a totaly different ROM. But you should wipe everything then before restoring the update: boot, system, cache, delviks cache,.. except for the radio and recovery of course.
@houdini1er: boot into the bootloader, go into recovery, whipe everything that is suggested in the fist post an try to apply the "update zip from sdcard".
I have reboot et launch the update via recovery, it work perfect now.
Thanks