I'm with Santam and I normally contact them every year to adjust the vehicles insured value, which is the retail value. They dont do this themselves, so you must contact them.
Also note that vehicles that are bought late in the year (e.g Dec) normally gets registered in the new year, so a vehicle you consider a 2008 model was actually manufactured in 2007. Your policy normally states "Year of Manufacturing", not "Year of (first)Registration" so even though you would like it to be a 2008 model, it should be insured as a 2007 model, hence lowering your premiums. In the event of an accident, the assessor will, and more than likely, pick up the correct date of manufacture and pay out accordingly i.e. the value of a 2007 and not 2008, even though you have been paying premiums on a 2008 model.