A 48-year-old man with bipolar I disorder has been stable on lithium 750 mg daily (serum level 0.8 mEq/L) for 2 years. He presents with a 3-week history of polyuria (4–5 L/day), polydipsia, and mild cognitive slowing. His serum sodium is 148 mEq/L (normal 135–145), serum creatinine is 1.4 mg/dL (baseline 0.9), and urine osmolality is 180 mOsm/kg (normal >600 after fluid restriction). Serum lithium level is 0.9 mEq/L. What is the most appropriate next step in management?