This is a 12-week, randomized, parallel-group, open labeled, flexible-dose study. The patients will be randomized to the risperidone-continuation group in which they continue to receive risperidone, or to the paliperidone-switch group in which they are switched from risperidone to paliperidone. In the paliperidone-switch group, risperidone will be tapered off during the first 4 weeks of the study, while paliperidone is titrated simultaneously. The doses of both drugs will be adjusted according to the clinical judgment of each research psychiatrist, within 6 mg/day of risperidone and 12 mg/day of paliperidone.