A 52-year-old man with osteoporosis presents 3 days after a fall on a flexed wrist. X-rays show a Smith fracture (volar angulation 28°) with minimal comminution. He underwent closed reduction 2 days ago and was placed in an above-elbow cast. Today's X-ray shows loss of reduction with recurrence of volar angulation (24°). Neurovascular examination remains intact. What is the most appropriate next step?