Currently, wind energy, a reliable, affordable, and clean energy source, contributes to 16% of Europe’s electricity. A typical modern wind turbine design lifespan is 20 years. In European Union countries, the number of wind turbines reaching 20 years or older will become significant beyond 2025. This research study presents a methodology aiming to upgrade rotor blades for existing wind turbines to extend the turbine life. This methodology employs blade element momentum theory, finite element analysis, genetic algorithm, and direct screen methods to optimise the blade external geometry and structural design, with the main objective to increase the blade power capture efficiency and enhance its structural performance. Meanwhile, the compatibility between the blade and the existing rotor of the wind turbine is considered during the optimisation. By applying this methodology to a 225 kW wind turbine, an optimal blade, which is compatible with the turbine hub, is proposed with the assistance of physical testing data. The optimised blade, which benefits from high-performance carbon-fibre composite material and layup optimisation, has a reduced tip deflection and self-weight of 48% and 31%, respectively, resulting in a significant reduction in resources, while improving its structural performance. In addition, for the optimised blade, there is an improvement in the power production of approximately 10.5% at a wind speed of 11 m/s, which results in an increase of over 4.2% in average annual power production compared to the existing turbine, without changing the blade length. Furthermore, an advanced aero-elastic-based simulation is conducted to ensure the changes made to the blade can guarantee an operation life of at least 20 years, which is equivalent to that of the reference blade.