This is a multicenter, randomized, open-label, non-inferiority clinical trial designed to evaluate whether downgraded surgery-guided by post-immunotherapy tumor boundaries-can achieve comparable 3-year overall survival (OS) to standard surgery in patients with Stage III-IVa locally advanced head and neck squamous cell carcinoma (HNSCC) who have responded to neoadjuvant immunotherapy. A total of 356 patients will be randomized 1:1 to receive either downgraded or standard surgery, followed by risk-adapted adjuvant therapy. The primary endpoint is 3-year OS, with secondary endpoints including disease-free survival, quality of life, complication rates, and cost-effectiveness. The study hypothesizes that downgraded surgery will preserve organ function and quality of life without compromising survival outcomes.