A 30-year-old woman with bipolar I disorder has been euthymic on valproate 1,000 mg daily (level 85 µg/mL) for 2 years. She presents to her PMHNP and reports that she and her partner are planning to conceive within the next 6 months. She has had two manic episodes and one depressive episode in the past, all requiring hospitalization. She is currently taking folate 1 mg daily. Her most recent hepatic panel and CBC are within normal limits. Which of the following is the most appropriate pharmacologic recommendation regarding her mood stabilizer?