A 42-year-old woman with a history of atopy presents with episodic wheeze and shortness of breath, particularly at night and when exposed to cats. She uses salbutamol inhaler up to three times per week. Spirometry confirms reversible airflow obstruction. She is currently on short-acting beta2 agonist (SABA) alone. According to the Australian Asthma Handbook, what is the most appropriate change to her regular therapy?