110-9 & 10 state that you must provide for the fault available at the terminals. The answer to your question is no, putting in 42k AIC breakers or fuses if the available fault current is 42 or less, would not violate the code.
The only question here is: Have the 42k AIC units been examined by UL or some other NRTL for use with the existing gear? The questions arise over mounting methods, positioning, etc.. If they have, you should have a compliant installation.
It is a question you may want to forward to the original designer. With luck, they did some sort of short circuit analysis at the time. Don't forget, in a large installation incoming AIC is only part of the story, in a fault, surge, etc., you do have the starting current of motors, inrush of transformers, and a large list of other items to consider.
