I guess it depends on the local office (ASC), while I am not at the F/P stage in the n-400 process yet, during the I-485/AOS stage I was not able to get the F/P done early.
The security person at the door would not let me in even though the place was nearly empty since my apptmnt was not due yet. I had friends who were assigned to a different ASC in bay area who were able to easily go as soon as they got the F/P notice and get it done early. So I would recommend that if you have the time just go and try, the worst case is that you'll still have to go back on your apptmnt date to do it if you are not successful in first attempt.