Yes, you can change the I-20 after you have booked your visa slot but have not still attended the interview. all you need to do is transfer the SEVIS from University A to University B. You do not need to pay the SEVIS fee again for the new university. Once you have a new SEVIS number then you would have to fill out a new DS-160 form for your new university. When both of these are done then you are required to just log in to your account on the CGI Federal website and change your DS -160 confirmation number and SEVIS ID. You will not have to pay the visa fee again. To know about the process in detail click here!