Well, I explained all my experience here:
https://www.mobileread.com/forums/sh...5&postcount=13
This is all we know, and it is the best known method to be done so far regarding OTA block.
Regular precautions apply - if you see a partial update file, delete it.
IF Kindle upgrades to new FW anyway - make sure NOT to factory reset OR update again before you restore JB and downgrade.