Good find! Yes, you can put a bigger ro
membrane in there. That looks like that's a standard size housing. If the old membrane was never used, it should be OK. Try it. I would get a TDS
meter too. I would also add a few other cartridges. If your going to be using the storage tank for drinking, it's nice to have a little inline carbon filter before the water goes into the tank. I would also add a DI
cart for your tank water.