Most European district heating (DH) systems face an urgent need to transform into climate-neutral networks in order to meet evolving energy efficiency standards. A key element of this transition is the integration of low-grade renewable energy and waste heat sources, which requires a systematic reduction in operational network temperatures. This paper presents a comprehensive methodology for assessing and optimizing DH network performance, using the 2nd-generation DH system in Opole, Poland, as a representative case study. The research combines high-resolution operational data from SCADA and GIS systems with advanced hydraulic modelling to create a digital twin of the network. The calculation scenarios include an analysis of the potential for lowering network temperatures, optimization of pumping station locations, decomposition of the system into supply zones, and development of a control strategy. Scenarios for network operation with multiple heat sources are analysed, including verification of the possibility of moving away from traditional control tables, which are still used in many district heating systems. Further scenario analyses explore the integration of distributed waste heat sources. The results show that, with targeted reconstruction of critical network bottlenecks, the system could operate at 90/47.5°C, effectively achieving the 4th-generation DH (4GDH) standard. It is demonstrated that traditional regulation curves of 120/70°C often lead to overheating. By establishing a new control curve based on actual system performance, stable operation at 115/65°C was achieved without capital investment. The study concludes that lowering network temperatures is a multi-level process involving hydraulic optimization, bottleneck elimination, and strategic consumer-side engagement. The proposed methodology provides a universal framework for DH companies to plan decarbonization strategies for existing 2nd-generation systems.