Even if the TDS reduction is OK, if you allowed a wet RO membrane to go unused long enough to call it "old," its likely the membrane is biofouled. Remember that membrane was partially submerged/marinating in dechlorinated water, and at least some of the membrane has dried after being wetted. I would not drink the water produced by that membrane. I would need some additional info to give you definitive advice, but in the absence of that, I'd replace it.
Russ