Just to add a wee bit to be really clear: the point of the load rating for wheels and tyres is to make sure that they do not become the weak link in an OVERALL load-rated vehicle, ie the vehicle and its components (including axle, shocks, brakes etc) has all been designed to handle that weight. The wheels and tyres are obviously things that can be changed quite easily and could therefore be changed to something that couldn't handle the load in the absence of the load rating system's guidance (someone ignorant could still ignore it obviously!). Hence the load rating to keep the system capable of that load capacity.
So there is no point in just changing the wheels. You would need to make sure that the complete system is capable.
And not just carrying weight but also stopping weight (momentum): it's why for instance T32 vans have bigger brakes capable of stopping the vehicle with 3.2 tonnes of gross weight.
As you've discovered now I think uprating needs to consider the whole vehicle system...!