Have noticed quite a few people saying that they did a walk-in for their Fingerprints prior to their appointment date. I wanted to do a walk-in because my scheduled FP date is 12/23/11 and I may be travelling that day. However when I went to the Detroit ASC, they refused and said that if I could not make it on scheduled date, I will need to re-schedule with the USCIS and then it will be a date later than the appointment date.