OK, a little schooling is in order.
First off prefilters and carbons have very little to absolutely nothing to do with TDS. Think about it, they are in the 10, 5, 1 or maybe 0.5 micron ranges. Thats suspended solids.
TDS or dissolved solids are in the 0.0001 micron range, many many times smaller than those are capable of removing.
You CANNOT use TDS as a gauge or measure on when to change prefilters and carbons, theye serve two different purposes. TSS (sediment, particulates, colloidal materials, silt etc) is removed by the prefilter and carbon along with chlorine and some VOCs. They are there solely to protect the expensive RO membrane whose job it is to remove 90-98% of the TDS with the DI resin getting the rest.

What you need to understand with TDS is if any is present, its possible you have more than you know. Weakly ionized substances like nitrates, phosphates and silicates are just tha, they contain very weakly chaged ions so do not measure well on a TDS meter. You can have high silicates and phosphates in the treated water and not even know it. Another very very important point is DI resins starte releasing these weakly ionized substances even BEFORE it is exhausted. That means when you start seeing 1 or 2 TDS up for 0 TDS you are starting to release contaminants the DI resin has attracted over time and put it back in the treated RO/DI water. Since they are weakly ionized you may not see them and again they will not test well on a TDS meter or hobbyist grade test kits since it is ultrapure water, but rest assured they are present if they were in the tap water.

You always change prefilters and carbon blocks every 6 bmonths like clockwork. At this same time it is advised you disinfect the system with household bleach following the directions I have posted many times to reduce the chances of virus and bacteria growth. You change the RO membrane when it is no longer cost effective to keep changing DI resins frequently and you change the DI resin when it is anyting other than 0 TDS on a regular basis.