I modified my 3458A to reduce the LTZ1000 temperature, replaced the NVRAMs, plus a few capacitors before returning to Keysight for calibration. They just calibrated & adjusted it before returning it with a 12 month warranty. The warranty was part of a special offer they did about 5(?) years ago.
I sent another one to them in 2011 that I had re-capped the PSU, fitted new NVRAMs in sockets, new RIFA caps, plus new mains filter. No complaints this time either although they made it really difficult to deal with them by insisting that I had to prove I was not a consumer. This, of course, after they had sold me a bunch of new instruments a while before & were completely willing to give me a substantial trade in deal on a load of junk.
The last time I sent my 3458A for calibration in November 2023, they did not adjust it even though it was outside the limits. My 34470A was within limits but was adjusted automatically for optimum performance.