A 55-year-old woman with bullous pemphigoid has been on oral prednisolone 40 mg/day for 8 weeks. She has achieved 80% clearance of lesions and is now being tapered. However, after reducing to 30 mg/day, new blisters appear within 2 weeks. Repeat direct immunofluorescence confirms persistent linear IgG and C3 at the basement membrane. What is the most appropriate next step?