KS needed to keep all versions since 1989 compatible, and especially the specification must not change.
I learnt that the reason for that is, that the 3458A is mainly intended for industrial use, i.e. inside manufacturing and testing lines at elevated temperatures, and these production lines are validated / homologized /audited. If you would change the 3458A specification, all these manufacturing lines would have to be re-audited, if an instrument would be replaced.
HP did not even improve / adapt the specifications, when several very obvious changes were made in the past, like the much better 40kOhm reference resistor in about 1995.
So although the new Black Edition really has many improvements and redesigns, formally / legally it's fully backward compatible.
For the very same reason, firmware is unchanged, as well as the interface.
On our Metrology Meeting in June, the KS representative already told us, that the DCV noise was halved, and that the DCI ranges are about 10 times more stable, and that the instrument is more stable overall.
Maybe the engineering group in charge for this redesign used better shunt resistors, probably similar to the 8085A, from Vishay.
Then, all these terminated components, like the EL2008 comparator had to be replaced by other, improved circuits and components. All that cannot be done by simply replacing through hole components by SMD, you have to completely redesign everything.
If you think about that further, 8051 and 68000 processors are also obsolete, and a compatible, modern solution had to be chosen.. not difficult to imagine how this was accomplished, and why now the 128kB memory is included.
Frank