I followed the instructions but it didn't work first time. I bit the bullet and stopped the update. I ran it a second time and it worked ok. It may have been a loss of connection between my laptop and the rack unit.
I think the first time, while the Line 6 updater was saying "update in progress" the display on the rack unit was still in normal amp mode, so I don't think the update began on the unit the first time - so the software was probably not corrupted/half installed.
Anyway it worked the 2nd time :-)
The loss of connection may have been something to do with trying to connect the rack unit with the HX edit software and the Updater software at the same time??