A 60-year-old man with bipolar I disorder has been maintained on lithium 1200 mg daily with stable levels around 0.8 mEq/L for 15 years. His most recent labs show a serum creatinine increase from 1.0 to 1.6 mg/dL, with an estimated GFR of 42 mL/min (previously 75). His lithium level at this visit is 1.2 mEq/L. He denies symptoms of toxicity. Which of the following is the most appropriate management?