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3.When are the biometrics, are they always before the green card expire?
Not always. You could send in your I-751 a day or two before you conditional Green Card expires, and you'd still be legal. If you did this, almost certainly the biometrics would be scheduled after GC expiration.
The biometrics are scheduled at the convenience of your local application support center (ASC). There are lots of these offices scattered around the country, with widely varying staffing levels, backlogs, and workloads, so it's hard to say when biometrics might be scheduled for you, and it's hard to use other people's experiences as a guide, unless they live in an area served by the same ASC. I'd expect it's often around a month or two after sending the I-751, but I wouldn't bet on a strong level of consistency.
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5. Can you reschedule biometrics an appointment?
When you receive the appointment notice, see it for instructions about rescheduling. Again, procedures vary from one ASC to the next, so it's hard to say what the procedure will be for your ASC.