A 28-year-old woman with generalized tonic-clonic seizures has been seizure-free for 2 years on phenytoin monotherapy. She now presents with plans to conceive. Her current phenytoin level is 15 µg/mL (therapeutic range 10–20 µg/mL). On examination, she has mild gingival hyperplasia and coarse facial features. What is the most appropriate management regarding her antiepileptic therapy?