The DI resin is always used up over time, so replacing the cartridge is not unique. Whether you buy a cartridge or simply resin that you pack into the plastic cartridge yourself, that's really the question.
If you want to buy resin in bulk, be sure to keep it sealed so it can't dry out. Some vendors offer individual packets, so you use the entire envelope's worth each time.
Dennis tried to recharge some resin a few months ago, and had a thread about it. You save the used resin, and soak it in lye and acid to recharge it.
