A 58-year-old man with long-standing hypertension and type 2 diabetes mellitus presents for follow-up. His blood pressure was previously poorly controlled on lisinopril, so his primary care physician added amlodipine 10 mg daily six weeks ago. He now complains of persistent palpitations and resting tachycardia (heart rate 105/min), though his blood pressure has improved to 130/78 mm Hg. Physical exam shows mild dependent lower extremity edema. Laboratory studies are unremarkable. Which of the following changes to his antihypertensive regimen is most appropriate to reduce reflex tachycardia while maintaining blood pressure control?