I still think you are looking at this the wrong way, maybe even AHJ you have mentioned is as well. You are not very likely to even need more than a 22-25kA rated breaker as the main. You would need one heck of a supply transformer and have very short run of conductors to it to even have over that level of current available. I really doubt your existing main breaker was 100kAIC rated, there has to be some misunderstanding on what you are seeing on that. 42kAIC rating isn't even all that likely on 120/240 single phase, you just don't run into a transformer that can put that much fault current out, even right at the output terminals, hardly ever.
Where are you going to run into 42kA+ IC rating needed? Industrial, heavy commercial, with transformers of at least 1000 kVA but even then, length and size of conductor between source and the switch/breaker in question will potentially knock down the available fault current at the switch vs at transformer secondary terminals. Get over 50 feet or so of conductor and it likely is low enough 25kA devices are suitable.