A 58-year-old man is being treated with unfractionated heparin for a proximal deep vein thrombosis. On day 7, his platelet count has fallen from 224 to 86 × 10⁹/L (reference range 150–400), and he has developed a painful swollen arm around a new intravenous cannula. There is no active bleeding. Which one of the following is the most appropriate immediate management?