Do I need to replace all the oil in the gearbox? Or just fill up the reduced part?

Every time you do the replacement, please drain off all the oil and fill up with new one. The long-time operation of the gearbox is going to occur some iron powders. The powders are going to block the oil pipeline of the gearbox and causing the insufficiency of supplying enough lubricating oil. It is going to accelerate the wear of gears and bearings and shorten the component’s working life.