About 50% of all hydropower plants (HPPs) worldwide were originally commissioned more than 40 years ago, so that the advanced age of the fleet is a major concern across all continents, and especially in Europe. The retrofitting of HPPs can generate several benefits for production, flexibility, safety, management and environment. In this work, the benefits related to energy and flexibility were considered and quantified by conducting a large-scale assessment for the European Union and Europe, taking into account several retrofitting strategies: dam heightening, head loss reduction in waterways, increase of installed power in run-of-the-river and storage power plants, increase of annual inflow, increase of maximum efficiency and weighted efficiency of electro-mechanical equipment, start and stop improvement, digitalization and inflow forecast, floating photovoltaic and reservoir interconnection. For most of these strategies, an indicator of the additional capacity and/or annual production that could be obtained compared to the current conditions was calculated. Excluding site-specific strategies (e.g. installation of new parallel waterways, increase of withdrawals from existing intakes) the resulting compound value of the indicator is 10.2% for European Union and 12.2% for the whole Europe, plus 4-28.6 TWh achievable by interconnecting reservoirs. This suggests that the retrofitting of HPPs can generate significant benefits in terms of energy and flexibility, minimizing environmental impacts, and should be considered as an important element of both energy transition and water management policies.