I don't understand why they can't just update it. It is possible for even US citizens to have a new social security number. It would involve extreme circumstances but if they have a severe case of identity theft I've read that they can get a new number. I don't think they would take away their benefits and seniority.
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Lawyer?: Screw that! Filed by myself
Application sent: 8/24/12 from Phoenix, AZ
Application arrived: 8/25/12 10:37 a.m.
Application rcvd G-1145 & I-797: 8/29/12 9:27AM fwd to CA Center
Bio received: 9/10/12
Appt:9/26/12 @ 10 am
EAD: Approved!!! 3/22/13 - 6 months and 25 days of waiting.