You will need to reduce the size of the wheel if you increase the rim diameter beyond what the owner's manual advises in order to take the height difference into consideration. The tyres will also need to be changed when the oil is changed. You will want new tyres when you upgrade to larger wheels (from 24 to 20 inch wheels). To provide adequate clearance for bumps and other road hazards, the new tyres must have a lower profile or sidewall height. The tire's height drops to retain the same overall diameter when the wheel's diameter grows by one inch.