The update_*.bin is presumably only updated because they are model-locked.
As I said, I don't think Amazon would've changed the format of the reginfo file -- and all the update_*.bin does is
copy that file across (for the benefit of people who don't have USBNetwork or a terminal emulator working). You can repack the update for any model, using
KindleTool.
Worst comes to worst, the copied-over file won't succeed in faking the registration and you are back where you started.