No, it's not. Read your insurance documents: It will state something along the lines of (nicked from the current Admiral policy):
You (not just you Vik, everyone) agreed to this when you signed up, and failure to do so constitutes a breach of contract on your part. As such, this means that they do not have to cover you if you've broken the terms. This isn't rocket science, it's not a matter of opinion, it's not heresay: It's contract law, something that has been around since the dawn of time. It's black and white, plain and simple.
Now, the only possible thing you could argue about is whether uprated brake discs and pads count as a material change. If you don't think it does, then fine. Go to court and argue your case, with absolutely no proof other than your opinion, against a room full of incredible expensive solicitors who will make you look silly and quote a million examples of case law at you. Or you could just tell your insurance company, like you agreed to do in the first place and let them make the decision. If they will charge you a lot extra or won't cover you, then take your business elsewhere next year.
Just for absolute clarity, this wasn't meant as a rant against you Vik, so please don't take it personally. It just so happens that you wrote something very clear that I could quote easily.