Just as Oscar said, each SFP model has it's limits and if a standard 10 G LR has a low warning threshold of, say, -14 dBm, that's because that type of SFP will start to lose the signal if it goes below that value. The switch reads all values like RX/TX high/low warning and alarm thresholds from the SFP itself. If your SFPs have warnings or even alarms on low Rx, you need to either clean the fiber ends and all connections in the signal path (potentially also the SFP RX and TX lenses) with an optical cleaning pen (or, even better, a cassette for fiber ends). Using the correct patches (of good quality) is also vital.
If you really, really need to change the RX values, you need to program the SFP itself with very specialized equipment, and most SFPs even have that hard coded and it cannot be changed unless the factory does it.
