A 38-year-old woman is on day 2 post-op following total abdominal hysterectomy. She has been using IV morphine PCA (demand dose 1.5 mg, lockout 8 minutes, 4-hour limit 25 mg) but reports breakthrough pain between doses. Her pain scores are 6–7/10 despite frequent PCA demands. Vital signs: BP 118/76 mmHg, HR 88/min, RR 16/min, SpO₂ 97% on room air. She is alert, conversant, and denies nausea or dizziness. What is the most appropriate modification to her PCA regimen?