That max. rating change is a severe problem, and KS should have informed all the customers about that, directly.
The background is not clear at the moment, so 'damage' could mean everything, from slight component damages, until complete wreckage of the instrument.
Also this term '.. over the lifetime of the instrument' is totally fuzzy.
What happens if I use the instrument anyhow at 1000Vp, and a damage occurs, will KS repair / replace on their own cost?
Will they fix this problem on already sold units?
I started a request on the Keysight DMM forum, already.
During the calibration process, 500VDC and 500VAC were proposed as standard values for the 1kV ranges.
For DCV, 450V...1000V, and for ACV, 400V ... 750V would be accepted by the firmware.
Frank