Owner maintenance rules permit owners to remove, clean and reinstall spark plugs. Plugs are expensive at $25+ each, and they need to be treated with care. They also should be rotated in their firing positions to prevent eroding the electrodes in an irregular fashion.
You need to exchange the # 1 cylinder plug with the # 4 cylinder plug and the # 2 with the # 3 if you want to make sure of the change in polarity. It is also a good idea to exchange positions - top to bottom - to ensure that a minimum of lead fouling accumulates.
Also, idle at less than 1200 RPM tends to lower the plug's nose temperature below 900 F causing the fuel's lead scavenging agents to be ineffective in preventing lead fouling. Remember, idle mixtures are overly rich anyway, adding to the problem of lead-fouling.