I just upgraded 2 of my 5231-ze cams, things to note:
1) For both of the cams that I tested this on, the first time I tried the upgrade, it seemed to go through the steps but then within 10-15 seconds it rebooted the cam. On logging on to the cam the firmware was still the old version. So I did it again and this time I saw the upgrade progress bar and the cam updated as expected. No idea why it seemed to not perform the update on the first attempt, can only surmise that maybe it was preparing the cam for the upgrade?
2) Substream 1 and 2 settings are maintained after a restart, on the previous version of the firmware, substream 2 would always be disabled and substream 1 would always be set back to default values.
3) On some versions of the previous firmware the Video Detection option would get re-enabled after a restart which would cause havoc with my IVS rules as it would trigger the cams on any motion. With this version of the firmware the Video Detection setting is also maintain after a restart.
All in all I'm very happy with this version of the firmware and all the bugs that I had previously reported have been fixed
Going to leave this running on the 2 cams for a few days before committing to updating my other 5231-ze cams.
EDIT: Forgot to thank Andy for the update