I just replaced several on my 35 year old Morgan. Both the gate valves and thruhulls were bronze. I hit them with a liberal dose of WD-40, attached a crescent wrench to the body of the gate valve and tapped with a hammer. They came off surpisingly easily. Mine is a freshwater boat. If your thruhull is corroded, there is certainly a chance that you''ll destroy it. In that case, you probably needed a new thruhull anyway. Good idea to replace the gatevalves.