That's not the way it works. AT&T does things a little differently, which is why we always end up waiting for a leak or OTA specifically from AT&T. Flashing anything other than AT&T firmware on an AT&T phone almost always guarantees a brick. The secure version of a bootloader, regardless of carrier basically claims what class the bootloader is in. From what you said, the new 4.1.2 FXZ has secure version 03, so once you flash that you can't downgrade back to a secure version lower than what you have.i've tested both versions of ATT FXZ and CHINA 4.1.2 FXZ
ATT has low score (secure version 02) than CN 4.1.2 (secure version 03)
does it mean that we (ATT) cant flash this FXZ ?
For example (I don't know the versions right off, I haven't looked)
Say Gingerbread has a secure version of 02
and ICS has a secure version of 02
and JB has 03
You can upgrade from GB to ICS, and downgrade to GB because they have the same secure version, but once you upgrade to JB you're in a new class and can only stay within a secure version 03 bootloader, or go higher, no more 02.