I say try your best to reschedule your test. If not, just go late. The member on here who did a walk-in said there was a guy who came in one day late and they still did his biometrics before walk-ins. I'm sure they won't do anything if you're a half hour late.
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Date Sent: 8/15/12 from Los Angeles County | Date Arrived: 8/16/12 at Phoenix Lockbox
Service Center: Nebraska | i-797C Receipt Received: 8/23/12 | Biometrics: 9/12/12
Approved: 9/28/12 | EAD Received: 10/4/12 | Applied for SSN: 10/5/12
Received SSN: 10/25/12| CA ID: 11/3/12 | CA DL: 11/8/12