Quoting Can I Enter on a Visitor Visa (B visa) and Change Status to an Exchange Visitor Program (J visa)?
If you want an earlier entry in the U.S. (more than 30 days prior to the course start date), you must qualify for, and obtain a visitor visa; however, this is strongly discouraged. If you travel to the U.S. on a visitor visa, before beginning an exchange program, you must obtain a change of visa classification from the B status to that of J. You must file Form I-539, Application for Change of Nonimmigrant Status, with application fee, and also submit the required Form DS-2019 to the DHS office where the application is made. Please be aware that you cannot start your exchange visitor program until the change of status is approved, and therefore in view of the processing time to your change status in the U.S., you may be in danger of missing your entire exchange program waiting approval of change of status.