A 52-year-old man with long-standing type 2 diabetes and hypertension is reviewed in general practice. His eGFR is 60 mL/min/1.73 m² and urinary albumin-creatinine ratio (UACR) is 8 mg/mmol (elevated). He is currently taking a calcium channel blocker for blood pressure. According to Australian CKD guidance, which change in therapy is most appropriate to slow progression of diabetic kidney disease?