If you look at Sennheisers own FR graphs from early HD800s to recent ones. They have changed them. Consistently for years, the frequency plots showed a hump at 6kHz, believed to be the primary reason some didn't like them for pushed treble. They also were criticised for having a bass that was too light. The latest graphs in the last few months have consistently shown a removal of the 6kHz hump and a stronger bass. Changes in measuring technique? perhaps....but as it is so consistent and in-line with what consumers would have wanted, I think its more likely a "quiet" change.
Normally Audeze change the first 3 digits of the serial number for an "edition" change of a headphone. For example, early LCD-3s started 231. They had different materials used behind and infront of the driver (cloth, vs foam etc etc) and they had driver yield issues. When the production line stabilised they changed to 261. I would have thought if they had changed the drivers again, with fazor, they would start using 291....can anyone with a confirmed fazor version confirm their serial number prefix?