If my fire extinguisher has been partially discharged and a small amount of extinguishant has been used, do I need to have the extinguisher refilled?

Yes, all fire extinguishers must be refilled once they have been discharged. Fire extinguishers are designed to discharge their propellant gas once the valve stem has been pushed. Even if you only use a small amount of extinguishant the extinguisher will still degas. This is so you always know that the extinguisher is not half full when put back in place.