Technically, yes, you can replace a plastic nozzle with a brass nozzle. However, check that the spray pattern, radius, and flow rate are similar to the original nozzle to maintain even watering across the zone. Brass nozzles and plastic nozzles have very different precipitation rates, so if you put one brass nozzle on a zone full of plastic ones, you could end up with a large discrepancy in water flow across your lawn.