For what it's worth, I had a great chat with support on the phone this morning after having called in and left a message. They called be back within ~30 minutes.
They confirmed that the 'auto upgrade' flag is really only in effect for newer vera units shipped in the last year (actually I'm unsure on exactly the time period). I have one of the very first Vera 3s as well as a Vera Lite and a Vera2, none of which had the flag set (obviously on the vera2). I was able to downgrade from UI7 to UI5 (1.5.622) without issue, using the steps described above.
I have some devices to reconfigure now, but this gets me back up and mostly operational after UI7 unfortunately broke a few things such as my Leviton 4 button scene controllers.