The FP notice usually is issued about 1 to 3 weeks after USCIS receives the N-400. Then the FP appointment will normally be 1 to 3 weeks after that.
Once you have the FP notice, you can go for a walk-in before the appointment date and/or at a different location than the one specified on the notice. Walk-ins are not guaranteed; they'll decide on the spot at their discretion whether to allow the walk-in. Don't call ahead; they'll almost always say no on the phone. They decide whether to accept walk-ins on a moment-to-moment basis based on how busy they are. Just show up and see what happens.
When you go for a walk-in, just hand over the notice and act as if nothing is wrong. Don't say anything about the different date or location unless they say something about it. If they refuse you, go back another day or try a different location. Or try again later the same day; they are often very empty in the late afternoon.
