The support at www.ustraveldocs.com/in have clarified that U.S. visa applicants who have already paid the U.S. visa application fee at one of the designated HDFC bank and have a receipt with PIN can use this to book an interview using the new system from September 24, 2012. At this time, applicants are receiving an error that the code is invalid. Our understanding is that the existing PIN numbers will automatically be accepted from Monday, September 24, 2012 and applicants will not have to do anything more.