I mean if you pay for the "modification", and write the car off, I imagine you won't get your money back for AB and EAP, FSD etc. unless you told them about it in advance. That'd suck but wouldn't be the end of the world. If you're not okay with that risk, you'll probs want to tell them. You can probably play around with some online insurance comparison sites to find out if it adds more to your premium.