UCLL6 is android 4.0.4, which is the release after the known-affected firmware (see quote below)
to be on the safe side, DO NOT FACTORY RESET BEFORE FLASHING UCMD8.
Since you plan to move to CM, and your phone is currently functional, then your path forward is clear:
1) flash the ucmd8 firmware to get the latest bootloaders available for the i777. This will also leave any superbrick worries behind you.
2) root and cwm the phone
3) flash whatever firmware you want.
The AT&T ICS leaks leading up to the first official ICS release, UCLE5, had a very serious flaw in the kernel which caused several people with the I777 to lose their device due to eMMC chip damage. The distribution of that flawed kernel was fairly widespread, and affected a number of different devices. There were very few confirmed cases of eMMC damage with the UCLE5 release, but evidence indicates it may still have contained the flaw in some form. The current UCLL6 kernel appearantly fixes some issues that appeared in the previous 4.0.4 release, such as the sleep of death issue. It is unknown if the original eMMC cap erase flaw has been completely removed from this release. Therefore, it may be advisable to either not run this firmware, or use a modified, rooted version in which the kernel is replaced with a known safe kernel. Such a modified, rooted version can be downloaded from the Android Development forum for this phone. It is unknown at this time if the bone stock installation can be rooted.
Originally Posted by d-r0ck
the baseband version of the phone is i777UCLL6
Build #: IMM76D.CLL6
Currently on 4.0.4 stock
Does any of that info indicate superbrick bug?