You should apply to change your status before your program starts. Bear in mind that if you are in the United States on a B-1/B-2 visa, you cannot enroll in classes or begin studying until USCIS has approved your change of status. Make sure you allow enough time for USCIS to process this. If you haven’t heard from USCIS at least 15 days before you are due to start, you should get in touch with the DSO at your school.