Thans for the comments--glad to hear there is still humor in electronics, even if at someone else's expense.
However, several posts missed the point: my choice was between copying the contents of the NVSRAM (then loading on to the new chip) and just doing a full calibration on the 3458A once the new chips are installed. That's it. Yes, I could send it to Keysight for calibration, but how does that help someone who neither has the skill and gear to do an SRAM clone nor the funds to send to a cal lab?
Some readers (and perhaps a few posters) might have their 3458A in a controlled environment, fully caibrated to yield measurment to the full spec of a 'new' meter and up and running 24/7. That might be my goal, but falling short of perfection is not failure. Considering the comment "precision and accuracy," I would add "repeatability" as the feature that sets the 3458A apart. That is something I learned from a very smart guy on 3458A's who also was the source of my resistance calibration standards and a wealth of good advice.
For those who don't know, The major selling point of the 3458A back in 1989 was that it required only two standards to calibrate: 10VDC and 10kOhm. That was sufficient for all DC measures. The SCAL was needed only every 2 years and generally was done in a cal lab. The AC section of the 3458A--unlike my 34465A DMM--can measure RMS voltage over a range from 1Hz to 10MHz. As far as I have found in the manual, this capability is not a front panel setting, only GPIB, any help here? In any event, this measure is not on my 'must-haves' list. SCAL is needed to setup for any AC measure, hence my need for an alternative approach. BTW, the AC section is six and a half digits and who know what one should expect from an eBay thermal converter. Good luck measuring RMS with a 'heater and thermocouple' anywhere outside a temperature controlled, draft-free space.
Finally, the RMS voltage is being generated by the Siglent 2042X linked to a Rh 10MHz standard--far superior to the 3325A synthesizer--and the 10bit ADC o'scope is in the circuit to monitor and fine tune the RMS.
Here's my conclusion: disassembling the 3458A, removing the A5 board, unsoldering a few chips, adding sockets and new chips and putting it all back together will require a full calibration even if the SRAM cloning is successful. Add to this the uncertainty of 'component fondling' during assembly, thermal shock from the soldering and assistance from the family pet and a full calibration is the only option.