What happens if he never files this form or contacts USCIS to let them know of a change in address?
He should file form AR-11 ASAP. He can also do a change of address online at USCIS.gov. If his I-751 is approved without an interview then his green card will be send to his old address. If he is sent a notice of interview to the old address and he does not show up for the interview then his petition is automatically denied and he might be put into removal proceedings.
Tania Pham, Esq. Immigration and Nationality Law; 17555 Ventura Blvd, Suite 200 Encino, Ca 91301; Phone: 818-334-4313; email@example.com; www.olenderpham.com
This person should file the AR-11 at the USCIS website now. At the same time he should update his address in connection with the pending I-751, also done online. If this is not done and the I-751 is approved, the new green card will be mailed to the old address and returned as undeliverable. Also, the U.S. citizen spouse must file Form I-865 to update her new address in connection with the affidavit of support.
He could have problems later on for failure to comply with the requirements of immigration law.
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