Probably best be on the safe side. Pretty sure you're good if you're upgrading parts that are OEM and were available on your car, for example fitting alloys from a same generation model. Otherwise half the people driving used cars would be uninsured. For example my old Golf S came with steels stock but was sold to me with alloys and this wasn't an issue.
It's a bit of a grey area though. OEM alloys have to be declared? What about aero wiper blades? You got the original dealership options receipt for those alloys? You get the point.
I'm sure you could plead ignorance if they ever bought it up but not the smartest move.
