A 28-year-old woman with generalized tonic-clonic seizures has been seizure-free for 2 years on phenytoin monotherapy. She now presents to the outpatient clinic planning pregnancy. Her seizure control is excellent, but she is concerned about teratogenicity. On examination, she has mild gingival hyperplasia and coarse facial features. What is the most appropriate next step in her antiepileptic management?