[en] The most challenging type of reactors to convert to low-enriched fuel are high-flux research reactors, which, along with some spallation sources, are the most important neutron sources for modern neutron scattering experiments. Advanced Monte-Carlo computer codes are now available that make it possible to track neutrons from the reactor core, through neutron guides, to instruments and detectors. These 'virtual experiments' allow optimizing the performance of a research facility as a whole instead of the reactor alone. This article briefly reviews performance gains obtained for high-flux reactors during previous facility-upgrades. The Monte-Carlo code VITESS is used to compare results for typical neutron scattering experiments using obsolete versus state-of-the-art technologies. The analysis shows that performance gains due to instrument upgrades or neutron guide renewals dwarf potential neutron flux losses due to conversion to low-enriched fuel. Combined convert-and-upgrade strategies therefore offer unique opportunities for reactor operators and neutron instrument groups to significantly improve the overall performance of research facilities. (author)