Abstract: The escalating challenges of rapid industrialization and increasingly stringent environmental regulations have placed immense pressure on existing wastewater treatment plants (WWTPs), particularly those within industrial parks. These facilities often face the dual imperative of expanding their treatment capacity to accommodate rising influent volumes and simultaneously upgrading their processes to meet higher effluent quality standards, all while constrained by limited physical space. To address these critical issues, this study comprehensively evaluated an in-situ upgrading strategy for a WWTP in a major industrial park in Nanjing, China. The technical approach integrated a Moving Bed Biofilm Reactor (MBBR) into the traditional Anaerobic-Anoxic-Oxic (AAO) system to assess its feasibility and efficacy for significant capacity expansion and achieving Class Ⅳ surface water standards. The results demonstrated a rapid and successful system start-up, with a mature and stable biofilm layer established on the MBBR carriers within 30 days during favorable summer temperatures. Under a demanding hydraulic loading rate of 150% of the original design capacity (corresponding to an influent flow rate of 1.5 m³/h), the hybrid system exhibited exceptional resilience. During the warm season, the treated effluent consistently met all Class Ⅳ surface water quality standards. While low temperatures during the cold season exerted inhibitory effects on microbial activity, the MBBR-enhanced process demonstrated remarkable robustness: the average effluent chemical oxygen demand (COD) concentration only slightly exceeded the Class Ⅳ limit (30.87 ± 2.54 mg/L), whereas ammonia, total nitrogen, and total phosphorus concentrations all comfortably satisfied the stringent requirements. This outcome confirms that the MBBR-enhanced process effectively achieved the dual goals of in-situ capacity expansion and substantial effluent quality improvement for the complex industrial park wastewater. Analysis of the microbial community structure revealed a pronounced seasonal shift in community composition and assembly. During the warm season, the biofilm community exhibited significantly higher α-diversity, with a notable enrichment of various thermophilic heterotrophic bacteria. Conversely, the microbial community shifted dramatically in the cold season, with a clear enrichment of cold-tolerant microorganisms. Notably, the significant proliferation of the genus Nitrospira was crucial for sustaining effective nitrification under low-temperature conditions. The assembly of the microbial community in the warm season was predominantly governed by deterministic processes. In contrast, community assembly during the cold season shifted towards a greater influence of stochastic processes. Concurrently, microbial network analysis indicated that the interspecies interaction network became simplified in winter and spring, likely enhancing overall community resilience and stability. In conclusion, this study offers a robust and scalable solution applicable to WWTPs worldwide that are facing similar challenges of expansion and stringent effluent quality improvement.