A 70-year-old man with a 40-year history of chronic obstructive pulmonary disease (COPD) presents for routine follow-up. He reports stable dyspnea at rest but no acute exacerbations. Arterial blood gas analysis shows pH 7.37, PaCO2 60 mmHg (normal 40 mmHg), and HCO3– 36 mEq/L (normal 24 mEq/L). Which of the following renal adaptations is most responsible for the patient’s acid–base status?