Early FP
mparisbr,
Please go to the USCIS ASC (Application Service Center) any day before your FP is scheduled and tell them that you want to have your FP done earlier as you could not do it on the day scheduled. They usually would allow you to do the FP on that day itself. Otherwise, give a letter to the USCIS officer over there asking for a rescheduled date. They would sign in your appoinment letter with a different date.
Eventhough, usually they allow FP within 90 days (they said 83 days at my ASC) after the original scheduled date at MOST places, I would not just take it for granted. It is in your best interest to go in person and have an early FP or get a written confirmation for rescheduled date. If you do that, 1). you don't slow down your processing or 2). you comply with the FP notice that says failure to reschedule is considered abandonment of the application.
Good luck and let us know what happened.