A 72-year-old woman with hypertension and CKD stage 3a (eGFR 52 mL/min/1.73m²) is started on lisinopril 10 mg daily. Repeat serum creatinine measured 2 weeks later is 2.1 mg/dL (baseline 1.6 mg/dL). Serum potassium is 5.8 mEq/L (normal 3.5–5.0). What is the most appropriate next step?