I would like to see a reference for this from the country that you're flying in. The only references I have seen define catagory on Vref (based on stall speed at max landing weight).
APPROACH category is decided by the operator with the blessing of its regulator and is for STRAIGHT IN ONLY ,it is based on VAT at max landing weight and pilots can increase lets say from C to D in case of higher vref.
CIRCLING category is NOT based on APPROACH categore but on SPEED one is to fly the circle portion of the maneuver.
FAA allows pilots to increase but not decrease their APPROaCH category.
Check FAA docs in their website.