Can a used ECU from a Honda CBR250R be reprogrammed with different
parameters to increase performance?
If your intention is to improve performance go with a piggyback system that controls both fuelling and ignition timing. A piggyback system can be remapped yourself too an unlimited number of times.
A stock ECU remap can potentially damage your engine as it overrides several safety parameters put in place by the manufacturer.
Usually ECUs from factory are not tuned to the highest possible performance figure, but are tuned in balance of power, handling, longevity, maintainability, etc.
It should be possible to reprogram ECUs, but it's much more wise to use plug and play devices(Dynojet, Powertronic to name few), which are altering air/fuel ratio to increase performance, and you are always free to unplug them and get to the factory settings.