A 58-year-old woman with rheumatic mitral stenosis presents with fever, fatigue, and a new systolic murmur. Blood cultures grow Streptococcus viridans. Transoesophageal echocardiography (TEE) reveals a 2.5 cm vegetation on the mitral valve with moderate mitral regurgitation and no perivalvular abscess. She is on day 10 of IV penicillin G (4 million units Q4H). Today, she develops acute dyspnoea, orthopnoea, and a third heart sound. Chest X-ray shows bilateral pulmonary oedema. What is the most appropriate next step in management?