From my experience, my device has never updated on it's own.. it just downloads the latest update and intermittently nags you..
I've reformatted my emunand to 9.0.0-20U countless times in the last week, and manually upgraded back to 9.5.0-23U..
When you format your emunand, it looks like it clones whatever your sysnand is, once that completes, I usually fork the 2 at that point, removing the SD and wiping the sysnand then manually upgrading the emunand