Hold on before you do anything. How old is the unit, when were the filters last replaced, was it stored wet once the original owner removed it or were the membrane and DI resin allowed to dry out, what membrane does it have in it?
To answer your questions you need to do some additional testing first. Let the unit run for 30 minutes or so and let it stabilize. While this is going on get a squeaky clean drinking water glass with no water spots or soap residue on it. Now fill the glass with tap water and test the TDS then triple rinse the glass and TDS meter in DI or distilled water. Next fill it with RO only water before the DI, this is important, and again triple rinse both the glass and meter in DI water. Finally test the combined RO/DI water and one last triple rinse of the meter and glass in DI or distilled water and put them away clean for next time.
Now. look at the results. A good 75 GPD RO membrane should be giving you a 96 to 98% reduction in TDS from waht the tap TDS was. So based on your tap readings from before if you start with 479 you should be seeing a RO only TDS of no more than 20 but it should ideally be no more than 8 or 10 TDS. If you have a 100 GPD Dow membrane it is not as efficient as a 75 (90% vs 98%)so you may get a reading as high as 45 or 50 and in that case I would easily say get a new membrane even if it not bad, more on the reason why later.
Lets assume the RO membrane is 75 GPD and is functioning properly and your RO TDS is 9. If you are still seeing a reading at all from the DI and especially if it is higher than maybe 1/3 of that 9 or 3 TDS then the DI resin is in need of replacement right away. The reason is DI resin once it is exhausted can actually do much more harm than good by releasing all the ions it has attracted and often in huge releases much higher than it was to begin with initially. NEVER ever use DI that is releasing anything higher than a 2 or 3 as you have no idea what the makeup of that 2 or 3 is. Weakly ionized substances get released first and a few of them are silicates and phosphates both of which are bad.
The reason you want the most efficient membrane possible is a nice rule of thumb in the industry is for every 2% you increase the RO membrane efficiency, you DOUBLE the life of the DI resin, so it can be a huge cost savings over time and most of us plan to keep our units for years.
If the prefilter and carbon are more than 6 months old replace them regardless of how much water has been made as they are potential bacteria and virus hazards. If the membrane is function within guidelines keep it if not replace it. If the DI is showing any reading above 2 replace it or if it has been allowed to dry out replace it anyway, resin must stay sealed and moist and has a short shelf life.
