Beginning July 1, 2019, USCIS will no longer accept Form I-407, Record of Abandonment of Lawful Permanent Resident Status at international field offices by mail or in person.

Individuals who wish to voluntarily record abandonment of their lawful permanent resident (LPR) status must submit Form I-407 via mail to:

USCIS Eastern Forms Center
Attn: I-407 unit
124 Leroy Road
PO Box 567
Williston, VT 05495

We anticipate that processing times at the Eastern Forms Center, from receipt to completion, will be within 60 days, which does not include mailing time to or from outside of the United States.

In very rare circumstances, a U.S. embassy, U.S. consulate, or USCIS international field office may accept a Form I-407 in person if an individual needs immediate proof that they have abandoned LPR status.

(From USCIS)