I successfully used the loading.kwi file from the 5.1 Navi DVD to load and use the data from the 9.1 Navi update for a Gen 5 system today.
I found that the easiest way to make sure that the system completely reloaded was to pull the battery positive lead. The Radio 1 fuse disabled the screen, but the Navi was not powered down. The Radio 2 fuse didn't do it either. It was simple to simply pull the positive battery lead -- and then reset the clock in the car and reset the seat memory.
When the system reloads properly, you get a message saying that the DVD is being reviewed. I didn't have any problems utilizing a Verbatium DVD +R DL.